They have mini sponge filters from whisper for 2 gallons and I use the 5-gallon mechanical filter and strapped a home-made device to the outflow to slow the current and make it more comfortable for my betta.
And for heat, I went cheap and bout a mini from hagen. It's non-adjustible sumbersible, but it's working like a charm thusfar. You can buy mini submersibles that can be adjusted, but they are pricier and harder to find.