In S E Asia you can buy new fat bikes for US$250 to $300. I have one, I believe was manufactured in Taiwan, where many bikes sold in the US are made. It has aluminum frame and front suspension. The tires are 26 x 4.0. The rims are 4 inches wide. Many new models have 3 inch rims with 4 inch tires, which is all you need. Mine came with 9 by 3 speed, but many are 7 by 3 speed, which is all you need. Mine is very strong. I will be surprised if the frame ever breaks.
Some bikes made in China are not as good quality. Some have steel frames.
When I was in Australia, I noticed online sellers selling bikes at cheap prices. I did not want to buy one then, so did not investigate much.
I also noticed some people in Australia get a fat bike as a gift, and don't want it. Some sell for about half new price. So you might get lucky wherever you are in the world.