You can buy one in Belize City. Courts, Mirabs (downtown), Brodies (downtown) Expect to pay between $1500 Bz and $4K. They deliver it to the shipping company and then you need to track it down and get it delivered to your house.
I use my treadmill (not that you could tell by looking at me.) I find that the beach is too uneven and causes me back problems, the street is too busy, and dusty/muddy. AND lets not even talk about mosquitoes.