You won't find anything dedicated to MZ in the USA...
Graham's Motorcycles is in the UK.
Pull up the 660 forum and run a forum search for your topic. Lots of info here, just gotta dig to find it.
As you read, you'll see that a lot of Yamaha parts will fit the motor.
Just don't tell the parts guy you have an MZ..."

Uhhh, we don't have any parts for that"...
Just tell them you need an oil filter for a Yamaha xxx...
Good luck and welcome to the world of MZ.
Bill R