I just recommend that you try to get heaters, Its easiest if you look online for large tanks, I like divided tanks because it saves space and you can use one heater per more betta (like was previously stated) I found my luck on craigslist and also sometimes petco has the dollar per gallon sale, where you can get brand new big tanks cheap like a 40 gal =$40 and so on but the smallest would be a 10 gal. Adjustable heaters are best. I would scrub down that 10 gal you had, divide it 3/ or 4 ways, (i've done 4 you just have to keep up water changes and some people think its too small it gives each one about 1.5-2 gals ) and use your heater with maybe an air bubbler to circulate water if you don't want to buy a filter.
All suggestions that help. but for the long run if you don't want to rehome them,
(I've done that on here, the people are nice and your betta's will go to great homes I recommend it if you can't buy what you need for them) you should start looking/saving for necessary supplies to keep them happiest and healthiest
I think its more people get so fed up with seeing betta cruelty they try to stop it ASAP rather than actually wanting to attack you. Just comes off that way
I wish you luck and I hope your betta's do well!