When getting a 2x anamorphic attachment, you can basically choose between three categories:
- Designed for 35mm-movie-cameras (Chrétien's original Hypergonars, ...)
- Designed for projectors in cinemas (film formats: 35mm and larger - there also some for digital cinema projectors)
- Designed for home cinema ("home cinema" in its original meaning: film formats: 16mm and less)
AFAIK the first category never showed up on used market (as most likely all of these first generation attachments are now found in museums).
Second category: See warnings from the EOSHD-Anamorphic-Guide (even though I've also seen some 35mm-attachments on eBay where the focussing range started somewhere between 1 and 2m, so -when in good condition- you would only have to fight with their larger weight and size.)
Third category: IMHO the warnings from the EOSHD-Anamorphic-Guide don't apply here as these attachments are smaller... . However you'll also have to watch the focussing range and the weight. The main problem with these attachments is the vignetting that's mainly caused by their small rear diameter and the -in some cases- relatively long distance between front and rear lens.
There are also some "complete" lenses (anamorphic part integrated into a "normal" lens - either designed for 35mm-projectors or for 35mm-cameras and some digital cameras). However the ones designed for projectors can't be used on a camera. And the ones designed for cameras will cost your first born.