As the title states, not having luck getting 15.5-25 readily or cheap, 17.5-25 is plentiful and cheap-ish
Probably depends on your wheel width and if you have clearance for the wider tires.
Lots of clearance, but the rim width is undetermined without dismount. I believe the unit is a 80’s IT12 or smaller
A 15.5-25 should be on a 12" wide rim. A 17.5-25 should be on a 14" wide rim.
When looking at rim size for 14:00x 24 tires it seemed to depend on different manufacturers specs. There wasn't a real clear cut answer. If I recall there were 3 different width rims that were specified. Calling a tire shop specializing in industrial tires would probably be your best bet if your existing rims would work. I've read you can usually go 1 size bigger tire on a rim without much trouble.
I've run 17.5's on rims that were 15.5 from factory without any problems, but it never looked right.