Hi Bizarrius
I had a few 645's before switching to bronica (prefer 6x6)... Hopefully I can help you out: Places to get the finder, Ffordes are probably one of the best, but also try London Camera Exchange, they get them in quite often used and sell them very cheap at the mo... Jacobs photographic (London) will often have parts in and again service and price is superb, and both options will mail to where ever you are pretty cheaply.
As for the strap, I tried several quick cheap options when I had mine, and all were pretty awful, ended up buying Mamiya strap lugs and matching to an Op-Tech strap. Not the cheapest option, but rock solid and made it easy to carry even for whole days...
Hope this helps.
M