I recommend two options for grenades:
1. Plastic dummy grenades. No noise, but are dirt cheap (we found them at a grocery store in the toy department for $3 for 2).
We play 10' kill zone. If it lands within 10' of you , call yourself out.
2. Wait for the Airsoft Innovations grenade. It will be pricy, probably around $80, but it is 100% reuseable, and doesn't require any replacement parts. It shoots out 180 bbs in a 360' spread, effectively killing anyone in a 15'x15' room, 100% of the time. The timer is accurate, and is set for an even 3 seconds. It uses propane/Green gas as propulsion, and is the only grenade that I've used that actually works like a grenade is supposed to.