Yeah, given the safety thing, I would not buy a pre-built battery that wasn't built by someone that actually cares about it and the end-user (even if it's only so they don't get sued). (same for buying cells)
I cant' afford the shipping and battery cost, either, so I sympathize.
This method isn't for everyone, but if you want a cheaper battery that you can still trust, that you won't have to *completely* build yourself, your best bet is to buy used large-EV cell modules that use large-capacity cells (with cells designed for higher currents than you're actually going to use them at).
Some of them are made of bolt-together cells or cell modules, so are pretty easy to rearrange into whatever series/parallel configuration you need. But even those that are all welded together are still usable--if you get one that has more series cells than you need, just connect your main +/- taps to a subset of them. If you can fit them all on the bike or vehicle, just leave the rest there and connected, just not in use in your system. Then you have spares you can wire in later if you have to, without any pack disassembly. Or if there are enough, it can be a lighting or accessory pack.
Most used large-EV packs work perfectly fine and don't have bad cells, and often enough all the cells are even still identical in properties, so when used well within their limitations they will remain balanced without a BMS taking care of that (which is ideal, whether or not you use a BMS for safety shutoffs, etc).
The thing about large-EV packs is that the cells in them, and the manufacturing techniques to assemble them, will have undergone much more QC testing and defect elimination than pretty much any "ebike" battery you can just go out and buy.
Modules can be either the traction modules, meant to run the motor, or they can be the ones from the "starter" or accessory batteries. Mostly depends on the current you need out of them vs their capabilities / design intent for which is needed.
Sources for these vary around the world; if you have places like Batteryhookup and other recyclers, or local vehicle breakers, you can keep an eye out for the specific ones you need--it can take time to do, so if you're in a hurry for a pack it's not really ideal, but if you can wait, you may get a screaming deal on the ideal pack for your usage.
