Usually a framing store should have some acid 
free tape in stock. You may wish to try Aaron 
Brothers either that or a Mom & Pop shop. I know 
Michaels doesnt carry it so don't bother with 
them (they use acid free tape for certain framing 
orders but they won't share any with the public.) 
You may want to try an art store as well..... but 
usually a framing store is your best bet.