Do the tires have 507 indicated on them? If so, here are some choices:
http://www.biketiresdirect.com/searc...=true&ust=true

I like the looks of the Contis and the Maxxis. I like the slick treads of both Innovas, but they're so dang WIDE!

The Continentals are kinda skinny, so may not work so well if you've really wide rims.

I'd probably wait 'til your current tires are wasted before replacing them, however.

If your main concern is puncture resistance, you could go with slime or Mr. Tuffy.