Search on Amazon for : Hygger Submersible Aquarium Heater, Adjustable Fish Tank Heater Compact Reliable Betta Heater, Heat-Resistant Milky Quartz Made, with External Temp Controller,
The 50 watt which is what you need, is $27.99. If you want a cheap heater you won't be able to set the temperature - it will just always set itself at 78 degrees +/- 2 degrees. I don't like that at all. I want to be able to set my thermometer AND I WANT TO SET IT IN FARHENHEIT which is what this thermometer does. If you want quality and features in the fish world you will pay for it. Also get a small pump and aeration stone - some betta's love to play in the bubbles and it will keep the tank well oxygenated. Get him a marimiso ball for a toy - many enjoy batting them around.
Now you've got the bug for keeping fish you are in trouble now. I originally started with a $1500 budget for a 29 Acrylic aquarium (twice as expensive than glass but 1/2 the weight and 16x the strength but seriously hard to remove algae and it's starting to bow so I am going back to glass) and the best filter, heater, plants, solutions and fish money can buy. I'm now up over $3000 with two 29 gallon and one 5 gallon and I just ordered another 29 gallon after my Gourami grew far bigger than advertised and I'm having a heck of a time keeping ammonia down. Plus they just look crowded or "cosy" - so I'll be moving a number of those fish into the new tank and, of course, buying a few more smaller fish - LOVE the Bushnose pleco and the Dojo Placo and I've bought most of the Gourami's available and attractive. These guys grow to over 4 inches but don't have the bulk of a 4" Gourami (or 6" as some of mine are growing to). Don't want to risk over populating again.
Buy the best and buy exactly what you want or you won't be satisfied.