Hi there, I went to Spa for the 1st time not so long ago for the 24 Hours and we had the same problem, until that is we found a camp site 10 minutes down the road! It's very pleasant with clean facilities and there is a noise curfew too if you want to get plenty of peace in the evenings. The directions are pretty simple from the campsite to the circuit, just ask at the reception, they were more then happy to help. Here is the link;
http://www.campingspador.be/homepagegb.htm
Have a good one!
(If you have any other questions send me a PM and I will try and help best I can)