Hi hmen,
SLA batteries are not typically stocked in drug stores or Walmart, etc. (like Alkaline batteries).
You can try to see if your local Radio Shack has 'em:
http://www.radioshack.com/product/index.jsp?productId=3307546You need a 12V, 5Ah SLA (2 of them). The link above is for one made by Yuasa. These should be good batteries, but I would ask
them to test the terminal voltage for you. If it is below 12.5V, I wouldn't buy them because SLA batteries should not be stored
for long periods of time without being "topped off." A fully charged 12V SLA should be approx. 12.8V.
If you do get 'em, please make sure that you fully charge them after installing them - BEFORE you play the amp.
If you need to purchase online, the Power Sonic brand has been very good:
http://www.batterywholesale.com/battery-store/proddetail.html?prodID=228Hope this helps,
Vinnie