I'm assuming you have a square taper crank here, which I think is a safe bet for a Trek entry-level mountain bike.
You *can* get 73 mm square taper bottom brackets if you look hard... Nashbar has a few UN-53 models:
http://www.nashbar.com/profile.cfm?c...eid=&pagename=
Unfortunately, it looks like they're out of all but 73x110 right now. You might call and see when they'll get other sizes in. Performance Bike carries the higher-quality UN-73 model:
http://www.performancebike.com/shop/...tegory_ID=5140, again you might call to see when they'll get the right size in stock.
Harris Cyclery carries a variety of them:
http://sheldonbrown.com/harris/botto...s.html#shimano ... scroll down to "73 mm width". If you can afford it, I would go for the UN72/73 over the UN52/53, as they're lighter and more durable.
Any competent Local Bike Shop should be able to get you a 73x115 bottom bracket, either they'll pull it out of the parts bin or special order it. By the way, a 73 mm bottom bracket is identical to a 68 mm bottom bracket in terms of threading. The only difference is the width. There are various shenanigans that you can do to get a 68 mm bottom bracket to fit in a 73 mm shell, you may want to search the forum for these if you're feeling adventurous
PS- In case I wasn't clear, it's okay to replace a 73x116 BB with a slightly longer or shorter spindle, e.g. 115 or 118 mm. Those sizes are more common. You'll have to make a few adjustments to the front derailer after changing the spindle length, since the chainrings will be slightly closer or further from the frame. But that's pretty easy to do!