These parts are available at Inventables.com. Inventables sells them kits, be careful not to double order the sub items like bearings and spacers.
The 2.x Laser uses a water cooled sealed gas 40W C02 tube. You can use other power tubes as long as they fit (about 750mm max.) in the enclosure and the diameter does not exceed the size of the tube clamps (53mm max.). All of the viable tubes I know of come from China. The shipping is a big part of the cost some be sure to include that in your cost comparison. The tubes are very fragile and often arrive broken. I took me three tries to get a good tube. Most sellers understand this and will send another tube. Be sure to check out that policy first. Tubes have a shelf life and will burnout after around a 1000 hours of use. Do not buy an old tube at full price. These tube operate in the infrared range, so the beam is completely invisible.
You need to buy a power supply for your tube. These are usually sold for specific tube power levels. You should make sure the voltage and current are compatible with your tube. Be sure it is compatible with your line voltage. They are often 110VAC or 220VAC specific. Some are specific to 5V and 3.3V logic. Most tend to be 5V logic. The current sent to the tube is controlled via a pin on the control connector. Some power supplies are analog only and some are PWM. The PWM power supplies can usually handle either analog or PWM. Avoid the analog only versions. Be careful, the voltages on these can kill you.
Test your tube and power supply with a current meter. Slowly increase the power to your tube until you reach maximum tube current. Note that level and make sure you do not exceed it in use.
Most power supplies have a connector with the following connections various orders. Usually they are labeled only with with a single letter or two (H,L,5,IN,G,W)
Here is PDF of a power supply for reference
The required optics include 3 mirrors and a lens. You will need (2) mirror holders and (1) mirror/lens holder. These must be of a specific type in order to fit the 2.x laser. With a 40W or lower laser you can use any type of mirror, with the gold plated types typically being the cheapest. The lens needs to be a ZnSe type lens. You probably want a 55mm focal length lens. Very few materials are transparent to IR light. ZnSe is the most practical for this. Never touch the lens. The material poisonous and it will damage the lens. The easiest way to buy the optics is to buy a kit from LightObject.
Most high end laser have assist air systems. This blasts a jet of air along the path of the beam after the lens. This does two things for lasers at our level of power. It clears the smoke away from the beam. The smoke can block the full power of the beam from reaching the work piece. It also keeps smoke from reaching the lens. The smoke can be full of dirty, sticky and corrosive chemicals. An assist air system with a nozzle can prevent smoke from ruining your expensive lens. If particles reach the lens, they can be permanently fused in place by the power of the beam.
The total required airflow and pressure is not that high so large aquarium pumps and air brush pumps work well. Ebay is a good source. Look for one similar to the picture shown above (ActiveAqua 60W, 70L/min). You can also use any other form of compressor you might have but limit the flow or pressure to a reasonable value. Start with 15 PSI and work your way up.
Be sure that the compressor outputs clean and dry air. You do not want dirt or water getting on your lens. Do not let your pump suck up dust bunnies.
The laser cutting and engraving process creates a lot of smoke and fumes. Some of the fumes can be poisonous and most of them smell really bad. The smoke can also affect cutting by staining the materials and blocking the beam. You probably want at least several hundred CFM (cubic feet per minute) of air flow. The blower should be placed at the furthest end of the ducting (ideally outside). This makes sure that any leaks are on the low pressure side and leak into the duct rather than out. Long ducts and turns can quickly lower the overall flow. Centrifugal type blowers are better than circular fans when dealing with back pressure. Remember that the air sucked out of the room needs to be replaced with fresh air. This may affect your heating/cooling costs. Opening a window in the same room may help, do be sure you don't suck back in some of the same air. Some areas have restrictions on what can be exhausted and you may need to filter the air through a charcoal filter.
Cable Carriers (AKA drag chains, energy chains) There are a lot of sources for this. The cable carrier used for the gantry must have a tight bend radius or it will hit the cover. Hitting the cover probably will not affect operation, but it will make noise and eventually scratch the cover window. I used Microtrak cable carriers from KabelSchlepp. They fit perfectly and the mounting bolt pattern matches the holes in the standard 2.x parts. I used the 0132 series, but the 0130 series has links that open. They are a little more expensive, but easier to install. The Tygon tubing for the assist air is very sticky and hard to pull through the carrier. You want to get part that in first.
A good table surface has minimal flat surface area so that the beam can pass cleanly through the material and not reflect onto the underside. It allows smoke to clear out of the cut and be removed by the exhaust blower. The best results are from expanded aluminum honeycomb material and metal lattice (such as used in light grilles).