My team's bot this year runs pretty much only GT2 3mm and 5mm timing belts for just about everything.
The belts/pulleys that come in the KOP are known as Gates HTD 5mm, Gates GT2 3mm/5mm sizing are a different line from the same company that is newer. The are some what interchangeable (gate's site has info on this) but not recommended.
The "fastest" option if you have an CNC mill is with the 5mm HTD/GT2 pulleys you can just machine them out with an 3/32 endmill finish cut:
http://www.chiefdelphi.com/media/photos/38380 this allows you to also machine in a hex bore because 99.999% (minus AM) of the stuff you buy will not be in hex.
But commercial suppliers we like are:
https://sdp-si.com/eStore/ and
http://www.bbman.com/
SDP ships from NY and BBman shipped from IN, and lead teams are generally 1-2 days before shipping IF ALL items in a order ARE IN STOCK. For belts that are out of stock, lead times are in the weeks. None of the pulleys you get them these guys will come with a hex bore, you have to do it your stuff with a broach or mill (we got a local team to help us out there).
You can also use gears to gear up a CIM, the new AM Spinbox does this and AM has the gears with the right bores. Or chain even.
Good luck!