Are there any pet stores in your area? Or a Wal-Mart? Those places usually have small aquariums. 3 gallons is a good tank size, 2.5 gallon absolute minimum. The bigger the tank, the longer the fish will live, basically. A larger tank with a filter is easier to clean anyway (the water won't get dirty as quickly).
A filter with only a very mild current would be preferable. A strong filter can overwhelm a Betta with their long, heavy fins. They need an aquarium heater and thermometer. No sharp decor please: plastic or rough plants and decorations can ruin the Betta's delicate fins. Silk plants and smooth decoration are better.
If you're planning on getting a number of Bettas, it's easy to start with just one. You can always get another tank later, and you will soon have a collection on your hands as you gain more experience with them. (They're super easy to take care of).