The slipper pads are pretty easy: Associated part ASC9603 has a pair of pads, and those are what nearly everyone uses.
For the rest of the assembly you have some options. Of course we recommend our own XF 5530 slipper plates; combined with the ASC9605 slipper spring and a good 5-40 locknut (XF 6074 for a cheap pack of 6 locknuts) and you have the exact slipper assembly we provide in our K021 entire car kit. While the adjustment range is kind of small due to the weak spring, this slipper assembly is well-proven. Properly spaced (use a .010" axle shim as Chazz suggested) and adjusted, it should provide a very long service life. I'm not sure what you consider "worn out" - the plates will get slightly streaked/discolored through normal use; I only replace them if they get pitted or scratched from debris.
About a year ago Associated came out with their "V2" slipper. It features a bigger/stiffer spring which provides greater adjustment range. To put that together you'll need ASC7485 V2 slipper plates and ASC7486 V2 slipper spring, all still held on with a good 5-40 lock nut. This slipper has better tension and a bigger adjustment range, but probably the same service life as the assembly above.
Finally, if you want to go all-out, you can get Associated's vented V2 slipper plates (ASC7495). I think they're a little over-kill (double the price) for our short races, but they do look trick.
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