door installation question

I have bought premanufactured aluminum doors from Frank Bear. I need a spacer of 5/8" between the door and the wall. I have seen several people use a wooden spacer (shim) between the two, but I can not find 5/8" boards and do not have a planer. I thought I saw somewhere on the fourm someone used a gasket of some kind. Maybe rubber. What are your experiences with this, how well does it work and where can I locate some?
Thank you,
Greg
a.k.a. backwoodsman
Thank you,
Greg
a.k.a. backwoodsman