Yes you can run them in either mode.
Well, you do have some options. Yes the FET-3 will run them in unipolar but I'm not sure what torque you'll get, that would depend on the specs of the motor; the quoted torque may be for bipolar. Or use the Xylotex but keep the current limited to the 2.5A max. This will yield less torque also, but I'm not sure of the trads-offs between bipolar and unipolar for your motors.
If you feel you can build, there are a few more options. Embeddedtronics.com has bare boards for a 3A 55V micro stepping chopper drive. If you have electronics skills they are fairly easy to build. Kitsareus has some kits that may do ok, very close to the FET-3 design. I belive there is a bipolar drive kit, but I can't tell you how well it'll do. There are several threads for scrach building you may want to look at.
About power supplies, here again lots of options. Building is the cheapest, or using a PC powersupply could be used also. Ebay has some deals, just have a clear idea what you are looking for.
Bill