Dashrimpmonster, glad you made it here, where there isn't a norm of 3 GF in a 10G.
Depending if you have someone who can help you move a bigger tank / can have a bigger tank in your space, watching for thrift store and moving sales on line and around your neighborhood. Many people want to sell their tanks for a high-ish price and it doesn't sell and they have to move. Often it will list free just to get rid of it.
Of course always have adult(s) with you and public place, etc. for safety.
Adopt a Betta from someone getting rid of it, also often free, I see them at least a couple or more every month. Or like $5 for (dumb) bowl and foods and supplies, etc.
There are also home breeders which have so many fish after each breeding that they often also have too many sometimes, and sometimes they are just reasonably priced. Sometimes they are listed a bit expensively just to make sure that you're not going to use them for fighting, food , be a throw away fish (get tired of it and flush, ignore) and if you explain your awareness about care and fish keeping goals you can get a better price (and that you are a young person).
Also although meetings in person are limited, Aquarium Society groups are all over the world and promote the hobby of fish keeping, especially to youth. Our club always has special items for the youth only, at cheap prices and some free to promote the hobby for them. Here is a place which lists the major ones
https://fishlab.com/local-fish-and-aquarium-clubs/
Going to club meetings (when they resume) often gets contact with very wise, long time keepers / breeders who also share fish.
Best sizes for tanks in the small to medium group are 20G long, 29G long 30 or 40G breeder tank are all ideal for the swimming that fish generally love to do, due to the horizontal space.
Tmaier is on target with some great ideas for fish in a smaller tank.
Online groups can be connections for free and lower priced stuff as well. Also getting a parent approved side hustle can net you lots of spare fish money!