Clueless about where to get a spare wheel - Help!

Hi All - I'm a real newbie to caravanning and I need to get a new wheel for my Lunar Premier 516L(1994) as I have one that is coming over its age limit on the offside.

My handbook says I need  a 165 R13 LS25C 6Ply

Having looked around some sites like Tow Sure etc I can find 165 and 13 and 4 ply but none that match the exact requirements

Does anyone know where I can get an exact or how important it is to have an exact of what the handbook says?

Any help much appreciated!