There's a new field between Bozeman and Belgrade called Battleridge Paintball. It's about a 10 minute drive from downtown Bozeman, and 5 minutes from Belgrade.
The cost is $10 per day, or $50 for a 3 month membership, $150/year membership. With a membership, you can go anytime you want. I've gone out there for quite a few weekends this year already.
The fields range from traditional 5 man speedball to huge rec fields built out of tires, spools, and pallets. The large fields definitely don't play like a modern speedball field, more like woodsball.
There is also a lot of woodsball to play around here, you just have to find the right group. I wouldn't say there are any spectacular places to play woodsball around here, however. The Hyalite canyon area is a good place to go, but it's got too much undergrowth for my taste.