I went sponge filters on ALL of my aquariums and I have 2 HOB filters just in case I need them (one came with my new 5.5 gallon, the other is a penguin biowheel)
I got mine from my local World Of Wet Pets and 90% of the writing on it is in chinese, but it's almost identical to this one
Sponge filter
I like this one because it's 20 gallon approved, and it's tall.. some are short and fat.. up to you which one you use, but tall and skinny IMO is better :)
The internal filter is good, many use it, but there's no way to block the intake incase the betta needs less flow into it. The ones with the long uptake tube can at least have a sponge put on it or nylons (yeah, like panty hose)