Car tyres/wheels!

[Jackson74]

I have some fat wheels on my jeep 265 width/50 aspect/15" rim. I cannot for the life of me find any second hand tyres for this size. I'm thinking of getting all replaced with a different size, do these new wheels need to be the same aspect/rim size as I think you would be able to change the width and be ok. Sorry if this is a silly Q, but don't want to fork out on wheels that don't work. Regards!