The soles only fit the Crossfire SRS Off-road boots. They use these pins that turn (similar to dzus fasteners) and the bottom of the SRS boot is made specifically to accept them as are the replaceable soles. Its a slick set up.
They are excellent boots. Very comfortable and break-in of less than an hour (in my case). They are also
very expensive. You can look for used ones or closeouts of odd colors but they go really fast in the popular sizes. I like them so much that I have another pair of Crossfires (non-SRS) for dirt track which I bought very lightly used out of the BARF classifieds for about 1/3 the price of new. Deals are out there.
As I mentioned above, you can add a slider kit to existing boots or make your own. I've seen people use plastic cutting board material from Tap Plastics. The cutting board material can be very slippery to walk in. Don't want to fall walking around the paddock before you even get a chance to crash on the track.