Each boat is different. They take different size diameter and shape tubes I don't think you would get a decent match off tubes from a different boat. When you say 'the tubes aren't great' does that suggest they just look a bit shabby or they are completely shot? How old are the tubes and are they hypalon or PVC? A proper re-tube for a 3.8m rib will be in excess of £2000 I suspect. Maybe more.
Post some pics so some more experienced people have a chance to look and see what sort of state they are in