I'm pretty sure either the Pentax refconverter-M or the refconverter-A will work, the refconverter-A without any modification. More info at
Bojiodar Dimitrovs page. Also, the Olympus Varimagni is compatible. All the above are only available used. There are a ton of aftermarket, universal finders that come with adapters for various camera brands. The one I have is the Seagull with 2x magnification. Picked it up on eBay for $80 or so. Not as nicely made as the one I had for my Contax 35mm system, but it gets the job done.