Where can i buy the main battery?

Selling now, shipping next week. And only one user.

To the OP I think first you need to ask whether or not you want a new or refurbished battery. It doesn't make sense to spend $2.5k to $4k on a Toyota new installed battery if you are going to junk the car next year. In that case, a refurbished battery can be much cheaper and last a few months, maybe a year, maybe more depending on how lucky you get. If you are OK with that, and expect the car to be scrapped in the near future, that's your cheapest option.

If you want a new battery, then you only have a few options. All of which are more expensive than the band-aid style refurb packs.