I got mine from the USA via Ebay.
You have to watch the postage charges but I found a good looking one and the postage charge was VERY reasonable. When I won (I think my winning bid was about £4) I mentioned to the seller that I could do with a second one as my other ST70 was also missing the battery box and he replied that he had another which I could have for the same price. In total I think I paid about £20 for both battery-boxes & P&P and the best bit was that when they arrived one of them was in BRAND-NEW condition (the other one wasn't bad either) and it included the rectifier (didn't need it but nice it was there).
I sold the spare on Ebay and it covered all my costs (didn't really need another one).
