[toni-a]

The is more than 1 other manufacturers for suck focal length reducers.

It would be wise to pick one with a more flexible mount. 

Canon EOS mount would be a candidate, as to it M42 can be adapted with cheap adapter rings, and also Nikon F-mount, Leica R-mount, Olympus OM mount, Pentax PK-mount, Yashica/Zeiss C/Y mount. And Canon EOS mount itself of course.

Canon FD has a nice short flange distance, but it is harder to find adapters to FD mount.

When it comes to adapters, the you can find plenty of Chinese low cost adapters, as long as no optics included you will be just fine, you can even have adapters with AF confirm chip.

I will be ordering some staff via www.aliexpress.com, the prices for adapters  are dirt cheap ( same price as a piece of cake at a local cake shop) I guess it would be worth trying.