The diameter is fairly standard. If you're unsure of the length, pull the sleeve from one of the others and buy three (or five if upper sleeves are the same) the same length.
There are other parts of the flipper mech that affect performance. You may want to get a flipper kit to rebuild all five.
The sleeves aren't specific to Gott, so you can get them most places. The rebuild kits are specific, so you may have to go thru Steve/PBR to get them.