I also searched for invisible counters some time ago. And found one -
www.megatracker.com
It supplies an invisible counter for adult sites at fairly cheap prices. If traffic is below 25,000 hits per month, it only costs $10. If traffic is about 1,000,000 hits per month, it would cost $220. And it seems that if you sign-up for their hosting service (www.sexxxave.com), that they supposedly give out free invisible counters.