I replace the seals in my two Mamiya RB67 every few years. For those of you more adventurous, I found this dense foam sealing material at Michael's (a craft supply store on the left side of the pond). It comes in 1mm and 2mm thickness, and it is adhesive backed. I cut what I need from the master sheet (which is 8?x11 inches, or 216x280mm), remove the old seal, and place the new one in. Best thing is, these sheets, which will seal about 50 cameras, are a couple of bucks each. Cheap!
Brian