where to buy gt40 valve springs?

The problem is if you don't want to pay money to have your valve guide machined down you can only run single springs. Performance wise roller stuff that leaves you with the trick flow spring upgrade, ford racing, and crower that makes some stuff. What cam are you running? you need a lot more info than lift (open height, close height, open pressure, close pressure) to pick valve springs expecialy if your going to try and do it on the cheap. If you want to do it easlly, just pay the extra money and use the springs your cam recomends.

I'm running the Compcams 942 springs. They are just right for a powerband below 6500rpm and they have dampers. Search part CCA-942-16 from Summit ($64)

Trick flow also makes a nice set of springs. I have them on my car witha b-303 cam, they are rated for use up to .540 lift cams. Summit sells them in a kit that comes with shims, steel retainers and locks, and also comes with new valve seals. The kit is 133 bucks. They have worked excellent for me.