I know you said you wanted to buy one, but I'm always of the mind that having options is a good thing. If you don't mind crafting one yourself, the forum's shop has plenty of pack parts to get the details just right and there are a few threads with plans on constructing one out of varying materials. Venkman71's got some detailed plans,
and here are some plans I recently found to make a pack out of EVA foam floor mats
. That stuff's a wonder kid for costumers. I've heard good things about Benofkent's work
as well. Still have to put it together yourself, but the hard part's done for you which is getting the look right.
Also, I have a bit of advice you may take or leave. I say it because I've seen it: If you start using student loans for non-school things, it's gonna come back and bite you in the rump. Hard. That said, if the student loans are being used for Student Stuff™ and it's your own chump change getting sunk into a pack, have fun! I'm sure some of the other folks here can better direct you to prefabs.