SW, there are plenty of places to buy TV's in SP but mostly in Belize City. You may not like the price on TV's compared to the USA.
Mattresses also, but a tempurpedic adjustable bed, well I'm not so sure on that one. We decided to buy local when we moved down and for the same reasons. We have only brought down items with us when we come down from the states or with our kids and guests when they visit. It is a challenge though. I will say that more and more is becoming available in Belize and sometimes I am really surprised that I can find some things. Cost and sometimes quality is somewhat of an issue though.
On the Island you have Hummingbird, Wings, Caye Supply and many other stores where you can find some of the items you will need.
Mirabs, Bennyís, and a whole host of other big stores in Belize City will have items you want and they will arrange shipment out here. I had a neighbor come down and outfit her whole place in two weeks from a few stores in Belize City. I couldnít believe it. Mirabs supplied most of her furniture and sent some men with the furniture to set it all up. She asked for the store owner or manager I canít remember and bought most of her furniture from them. I have no idea on cost on that arrangement. Now my wife and I? Well letís say we donít move that fast and it took much longer.
Most stores will give you some kind of a discount both on the Island and in the city but you must ask. Good luck. I donít think I would want to suffer through that adventure again but we did have a few laughs. Shopping, like most men, is not my thing.
PS In addition there are plently of furniture builders who will make most anything for you if you have a picture or a plan out of wood. Just ask around for them. Because of the economy the last few years there are a few less on the Island but there are some and there are more on the main land.