What I would do is just pick up a set of junkyard L67 GTP injectors. You can probably pick a set up for around $60. Buying them brand new would cost you an arm and a leg. Next I would get them professionally cleaned and tested. They will test each injector's flow pattern to make sure they are all flowing equally. I've found prices for this service is around $20 per injector in the USA and about $30 per injector in Canada which is brutal!
Check
http://car-part.com/ for 2000 Pontiac Grand Prix Supercharged injectors. The website will cross reference the part with other model years and vehicles. You'll be able to see which ones are the cheapest near you.