If you cannot get a direct replacement for your gun’s unusual roller retainer, I would just run it without a retainer while I looked for a German or RCM MP5 bolt, or get something better- an M.A.D. MP5 bolt.
What I did on my Z-5RS is replace the factory bolt with a M.A.D. MP5 bolt. This got me 1) a far (FAR!) more precisely made bolt than what came out of Turkey, 2) a bolt which uses the more durable “wire” roller retainer, rather than the MKE bolt’s old-style “plate” roller retainer, and, most importantly 3) a VASTLY improved extractor design which won’t crap out when I want/need my gun to work.
I realize that $300 is a lot of money for a bolt, but in this case it really is worth it. I cannot say enough good things about the M.A.D. MP5 bolt. The quality of manufacturing and excellence of design are unparalleled. Not to mention that the M.A.D. bolts remedy the biggest engineering shortcoming of the MP5 design; the awful, weak and strange design of the extractor spring. Get yourself one before M.A.D. has to stop making them, because too many people won’t pay for quality items.