please don't buy a betta from wal-mart. I did because I felt sooooooooo sorry for this little guy. But I realized that buying him means more betta's will be shipped in to suffer.
But I couldn't leave him.... we made eye contact. lol
By far in my area, Petco has the best variety of bettas and they are generally healthier and more well kept than PetSmart and even my local Aquarium Store.
I have him in a 2.5 gallon Critter Keeper (the size is 'large' I wish they put gallons sizes on there) He has a heater and no filter. I do 100% water changes 3 times a week. I actually like changing out his tank (I like cleaning... I'm weird :] )