Real men ride 20".
But yeah, mountain bikes come in all sorts of variaties. You can't be expecting much for $100 unless it's used,I have over $1k into my 20". So I'd check out CL/garage sales if you haven't been told that 100 times already.
Also, check out
JensonUSA, they have a Closeout section that has had some MTN bikes go through. Trek, Giant, Schwinn, those are all good brands. Diamondback is a great brand too, but don't bother looking at online reviews, they've been caught redhanded 5 starring their own products.
Oh yeah, and in some towns, you can go to the police station, tell them you've had your mountain bike stolen, they'll ask for a general description, say a black mountain bike(or your preferable color), and then you can go see the bikes they have found...
P.S. this is what part of the alphabet would look like if Q and R were eliminated.