On my tandem axle trailer, I purchased a 14" 5 on 4-1/2, white steal modulal wheel from Tractor Supply (don't laugh, we have them in Texas and they are a great source for trailer parts) for $38, and used the best of my old Goodyear marathons as the spare. Mounted it on the trailer and covered it with a black vinyl cover. Worked great. Total investment less than $60 including cover and labor to mount and ballance.
On your single axle trialer, I think you need a 15" wheel with a 6 on 5.5 bolt pattern.