While I was taking down my instrument after rehearsal 3 days ago, the clamp's (I'm not sure what the name of the part is, but it's the clamp that locks the long joint and wing joint together) spring jumped out. Since the spring is super small, I couldn't find it. Without that spring, the wing joint and long joint will be loose and moving while I play, which is really annoying. Haven't had any luck in finding it. I asked my band director if he can order a new spring, but he said it'll be a hassle to order and wait for something that's really small. Is there any way I can replace the spring somehow? Or is there a music store that sells that specific spring?

You don't say what make of bassoon it is - Fox/Renard? If it is you've probably lost more than the spring & will need to obtain a complete body lock pin & spring. These are available on line from Fox or any repairer or store that specialises in bassoons is likely to have these in stock. http://www.foxproducts.com/Replacement-Pin-Spring-for-Body-Lock.item

Good luck.

Oh, my bad, I forgot to mention what brand it is. It's Renard by Fox (that's what it said on the bell)

Those pins are easy to break. Sometimes the tip come unscrewed & is lost along with the spring.