Here's the specs for your bike. I remember the IBOC series from Mongoose. Those were actually some quality frames.

It looks like you have a 1 1/8" steerer tube (most common in the last 15+ years). As for length, you'll have to measure your current fork from the crown to the top. If you have a hacksaw, file, and something to guide the saw straight, you can cut the new one yourself to match the old one.

What features do you want in a fork? Similar to what you have? Rebound adjustment? Lockout? What's your budget?