There's a whole system on
http://woot.com
Mr.Clean AutoDry Carwash System
Sprayer, starter filter, and soap for 2.99. Shipping is $5 more.
You can buy up to 3 systems; shipping stays at $5. So $13.97 for 3.
I use this from time to time, figured maybe someone else does too. The "Starter Filter" lasts for 3 washes or so, not as long as a full filter; there's more than enough soap for 5 though.
For the price it's a good buy.
Just sharing.