I am having trouble finding a fat tire inner tube 20" x 2.8 or 20" x 3.0 Please let me know if you have any idea where I can buy one or can you point me in the right direction. thank you.
You don't reveal where on the planet you are looking Bernard!
For example in Australia the main bicycle wholesalers have both 20x3 and 20x4 readily available. I'd imagine if you are in Europe or North America it would be similar, so you just need to ask your lbs to order it in for you. It's unlikely to be a stocked item as it's a slow mover.
A motorbike shop may also have them but they won't call it 20"! To them it will be about 14" or 15". Just show them your old one or the tyre.
Not a lot of people know that bike tyre sizes in inches are given by the approximate outer diameter, whereas motorbikes and vehicles go by the tyre inner diameter or wheel OD, hence yours is called approx 14" to them. HTH.