Dont get any bottom feeders. That's not fair to them.. They dont have enough surface on the bottom to go on.
Id stay away from other fish in this tank. It can be done, yes (though Id suggest you have a filter if youre gonna get any, Idk if it has one), but youre guaranteed to encounter issues. Bottom feeders dont have enough room and higher level swimming fish are where your betta needs to be, which means every time the betta comes up for air, its going to compete with or chase the other fish.
I know the tank is big, but the height will give you trouble. Great tank mates would be non-fish, like shrimp or snails.
Instead, maybe, of having fish in there other than the betta, consider planting it. It would look really, really great! You can do a lot in that tank, and considering its height, you can plant live plants that would both look lovely and give your betta a place to rest and relax thats close to the surface.
You dont *need to have biube products, per se. The silk plants petco makes would work well! Also, have you considered removing the air tube, to give the fish more room? The bubbler is not needed at all and provides the betta with nothing, since it breathes surface air.
If youre looking, petsmart and petco.com may have them, amazon.com usually has great deals, and there's always ebay.
For live plants (a worthy endeavor) try bobstropicalplants.com, sweetaquatics (though have trouble sometimes with late shipping..), plantedaquariumcentral.com and aquabid.com in the plants section.