However the aperture blades are stuck and oily from both sides. I managed to open the lens and clean the blades from both sides with alcohol. I reassembled the lens and its works just fine. However the oils seems to come back over time and the blades are stuck again. I disassembled the lens now for the third time; cleaned the blades and reassembled everything. It always works for a few days but then the oil comes back and glues the aperture blades back together. How can I permanently fix this issue?
Since it was for free I don't want to bring it to a service center and pay a lot of money for it (maybe I would let them fix it if they would fix it for 20$ max). I would prefer to fix the lens on my own. I'm interested in learning how to fix it.
You need to locate and clean the source of the oil. Most likely, you would have to disassemble the lens, clean out the old grease, and regrease the helicoids.
You should also use a nonpolar solvent (naphtha / mineral spirits) instead of alcohol.
Replacing the aperture assembly is not a viable option.
There is no listing for an aperture assembly for this lens on eBay.
The aperture assembly on vintage lenses is usually an inseparable part of the optical block or inner barrel.
Even if the aperture assembly could be replaced, the best you'd find is a donor lens, which would have the same or worse problems as your current lens. (Otherwise, it would be a replacement lens, not a donor.)
The old grease will keep running into mechanism. The only way to eliminate the problem is to do a complete CLA. It needs to be flushed and cleaned of all grease, then properly relubed with new grease. Since it is a 200mm it has alot of grease in its long helicoid. If you are not comfortable with a complate disassembly then you should sell it for parts and get another one. A CLA will cost more than buying a good replacement.