Ive just bought a 2005 a3 2.0 turbo. I want to do some mods like a big intercooler ect but not sure the best site to get bits from any suggestions?

 

There are a few places around - especially for the 2.0t.

Try:
https://www.ecstuning.com
https://034motorsport.com
https://www.moddedeuros.com
Audi Performance Parts & Repair Experts Porsche, VW Service & repair - PURE Motorsport, LLC

There are probably many more - just keep googling
Keep in mind that many sites don't sell parts for 2005 A3 models since the car officially didn't start selling until 2006 in the states (I know that in Germany it has been around a lot longer but not here). However if you have the FSI engine with the 2.0t - you can look at the 2006 /8p parts.