It is obtainable from eBay around $20~$50. As it is, mounted lens cannot achieve
infinity focus easily, nor easy to dismount the lens without taking out the whole thing.
The later is more problematic for real use, since you do not want to taking whole
thing and change the lens, this is the same as mounting C/Y lens with an adaptor.

You can cut a ditch on the EF mounting base metal so that you can mount/dismount
PB lenses. In order to achieve infinity focus, you may have to thin the base metal of
this adapter by 0.1 ~ 0.15mm. However the most of PB-lenses do cause 5D's mirror
to hit the rear lens protector or lens itself since 5d's mirror is too long even for some
of M42-lenses such as Flektogon 25/4, Pancolar 80/1.8, and EF-mount Rokkor 58/1.2.