I can't imagine that you'll get good results with used battery acid. You have no way of knowing the strength of the acid, and what's been done to it before you use it.
You MUST be able to find somewhere that will sell you new acid, perhaps a commercial auto supply store.
If you're stuck, try contacting our UK distributor and ask him if he knows a good source. He sells our anodizing kit, which requires the acid, so he must know somewhere.
Speed Demon Motorcycles
5 Overdale Walk
Whitehill, Hampshire GU35 9DR
UK
Phone: +44 (0) 1420 476145
Fax: +44 (0) 870 136 5288
caswell-europe@caswellplating.com