"Where are some good places to buy USB audio interfaces at good prices?"
Suggestion #1:
Do not, repeat, DO NOT, buy a USB interface.
Get a FIREWIRE audio interface if you have a MacBook Pro with a firewire port.
Firewire is far, FAR superior to USB for audio input/output, particularly where latency and recording multiple tracks are involved.
Suggestion #2:
I've tried both Presonus and Echo interfaces. Currently using an Echo AudioFire8 and it's very, very nice. Plenty of inputs, good mic pres, and excellent analog-to-digital conversion.
As you check out various interfaces (firewire, remember?), take a look at the Echo AudioFire4, as well.
Suggestion #3:
In many ways, your choice of mics will become more important than your interface. Build knowledge on them BEFORE you buy.
Suggestion #4:
There is a dedicated "digital audio" forum right here on macrumors.com - worth visiting.
Also consider visiting "gearslutz.com" for more info.
This man speaks the truth.
Also, ff you can at all foresee yourself pursuing this seriously down the road, you may find it beneficial to invest in Pro Tools or Logic. You also need to consider your goals and methods. Garage Band and Logic (which can be thought of as GarageBand's "bigger brother") are geared more towards music
creation, while Pro Tools has historically been more
recording-oriented. Both programs have added more of the "other" capability in recent years, but you can still see the underlying design philosophy in both. If most of your music will come from real instruments, you might prefer Pro Tools. Conversely, if you mostly want to accompany your guitar (or whatever) playing with electronically-generated music, you might find Logic to be more suited towards your needs.
Pro Tools does require that you use DigiDesign hardware, though*, so your best bet may be to get something like the Mbox 2 Pro**, which connects via FireWire, comes with Pro Tools, and is also compatible with GarageBand/Logic. I've used one with all of these applications and had great luck.
If you absolutely plan to never use Pro Tools, though, you can get better hardware for less money, since a good chunk of the Mbox price goes toward the software. But again, your mic choices and placement are going to make an order of magnitude more difference than your choice of audio interface, as long as you're not talking about something like an iMic.
For shopping and more detail advice, you may also want to check out
Sweetwater. They assign you a personal agent who will help you determine your needs and what gear would best suit you. These guys are people who've worked in the music industry, and they're not just out to upsell you; they've talked me into buying something cheaper on more than one occasion
Also, whatever you decide to buy, don't forget to look around to see if you can get educational discounts (if you're affiliated with an educational institution, of course) and/or a better deal on slightly-used gear.
* Except for Pro Tools M-Powered, which works with certain M-Audio interfaces, but I understand that this is only cost-effective if you already own one of these interfaces and want to add Pro Tools, since the software alone can cost almost as much as an Mbox+Pro Tools LE combination.
** Do NOT get the regular Mbox 2; in addition to all the other problems with USB recording that Fishrrman mentions, DigiDesign's USB drivers are buggy, worthless crap. If you only plan to use Pro Tools, you MAY be fine with a USB Mbox, but if you try to use it as a Core Audio device you're in for a world of hurt. Digi has known about the problem for ALMOST THREE YEARS, and only in the last few months have they released a kinda-sorta fix that only usually works for most people. And definitely don't get the original Mbox, since Digi discontinued support for it with the latest version of Pro Tools.
