There are lots of online hobby shops that carry rubber slicks. As mentioned for Sorex 36 tires they can be found used as they were the Reedy Race handouts. These are for pretty warm weather/track surfaces though and you might need something a little softer.
I own quite a few compound due to varying temperatures as I race outdoor on asphault. Sorex, TakeOff, and Jaco all make good tires and lately I've been running mostly Sorex as they are available and there has been a change with manufacturing for the TakeOff tires. The new Jaco on Prism wheels are still taking a while to hit inventory but have received really good reviews.
Most of my online shopping either goes to one of my LHS sites jphracing.com or amainhobbies.com who never let me down.