A heater for a 10 gallon tank will work well. You can find Tetra brand heaters for tanks that small, but it is not adjustable, and has a set temp of about 78F (+/- 2F). Eheim makes good heaters (I use Jager heaters in my SW tank), as does Aqueon (their Pro heaters are what I use in my FW tanks).
The smallest Aqueon Pro is 50 watts, good for tanks less than 20 gallons. They also have "mini heater" that doesn't appear to be adjustable (
Aqueon » Mini Heater | Products).
Eheim has an adjustable 25 watt heater for smaller tanks.
Products has info (click "technical data" tab for more info). They run around $25 (see
Eheim JAGER TruTemp Aquarium Heater - Bulk Reef Supply for one place that sells them).