I use this SITE . They have good prices on rechargeable batteries.
I selected 6v, 12 a/h batteries and they are $20.
Scroll down the page past the solar charges and they have twin packs for $34.
All prices include shipping and with Free Returns if it isn't what you want.

I bought an extra 12v for my DR driveway grader, and ended up using it for rockets.
It came with a handle for carrying and it weighed much less than a car size battery.
I think I paid $20 for that too, but that was several years ago.
It's got some good stuff, and they have some HF type junk too.
They have a nice, small gravity spray gun kit I plan on getting to paint rockets.
I had one before, but I had to sell it before I even opened the package due to life problems.