Turtles are inexpensive around $80 and you can get them quite small. Your tank needs to have a platform your turtle can get up onto and a filter.
Turtles are great pets, I had one as a kid
Frogs you can buy from specialty pet stores, I don't think your allowed to keep native frogs except green tree frogs & you need a license, they take alot of lighting, heating etc too.
Turtles are a bit more hardy prob a better option for a pet