Kensfish.com also has them for right around $20.
If you're going to go with a larger tank, you'll probably want a larger filter. Generally with axolotls you want something powerful that doesn't move a ton of water, because they don't like water flow. I use an Eheim classic series canister (2217) on one of my aquariums and can't wait to get one for my axolotl tank (the 40 gallon they're going into). If you want to spend a bit of money, they're amazing filters. I know people who've had them for close to 20 years and they still run, they're dead silent, and extremely simple to operate. But, the smallest one is around $75 (drs. foster & smith or Kensfish, both have similar prices), and they're rated for tanks much bigger than 30 gallons. The only cheaper canisters I can think of are HOT Magnums, and I wouldn't recommend one of those, as they don't do anything at all for biological filtration, which is really the most important part of filtration.
Sponge filters are WONDERFUL. I use them in a few tanks, and love them. However, they're about 99% biological filtration- they don't do much of anything for mechanical filtration. Most of the crud settles to the bottom of the tank, and the sponge only works on polishing fine particles- and the water does sparkle in tanks with sponge filters. And, if you use a sponge filter, you have to run a powerhead (which I wouldn't do with axoltols), or an air pump. If you run an air pump, that's noise, and you hear bubbles all the time, which will probably be louder than a standard HOB filter.