I just researched this pretty hard and after hearing the HyDrive cabs I liked them alot, but your taste may be different. Do you want new or used? If you want used there is/was a 115 and a 410 (HyDrive cabs) in the Amps for sale thread.
HX115 - $499 New & online or $350 used (in Amps for Sale)
HX410 - $400 used (in Amps for Sale)
Also, if you get an 8 ohm cab you can always add another 8 ohm cab down the road to get a total of 4 ohms and get the full 500 watts. Or just try to find a 4 ohm and get the 500 watts. I like the 8 ohm idea much better. Gives you room for more options down the road. I'm sure others here can tell you way better than I on this and other types of cabs.
Good luck with it.