Motedis based in Germany, cheapest supplier of aluminium profile and connecting pieces I have found so far.
I ordered from their UK site, but items were shipped from their German warehouse. work out your cutting list, then have it cut from 6m lengths to save money, I paid via paypal.
myhobby-CNC German Makerslide and profile supplier, again you can pay with paypal.
RobotDigg based in China, but the cheapest supplier of M4 or M5 T nuts I have found by far - $7.50 for 100, also cheap GT2, MXL belts and pulleys, payment via paypal.
Bearings, micro switches, drag chain, etc. - Ebay from China, payment via paypal, I bought 40 x 625zz bearings @ £1.61 for 10, 10 x micro switches for £1.48 and 1m of 10x10mm drag chain for £4.08, most of these suppliers list on all Ebay sites in a variety of currencies, most offer free shipping and prices are low enough that they will not attract customs charges.