I am appealing for some advice. Not so long ago I bought a Panasonic G7 and decided to get a Roxsen lens adapter with a Canon FD mount as I'd heard good things about second hand Canon FD lenses and their relative prices. Got my hands on a Canon FD 28mm for about £80 and I'm absolutely thrilled with it. Here is an example of some low light footage I shot with it a while ago: https://vimeo.com/195189918
As 28mm on a Micro Four thirds camera (the Roxsen adapter reduces the crop factor somewhat) is closer to 50mm my next step is to look for a nice ultra wide, to get nice wide angles - to which I then found out that the Canon Ultra Wides are much rarer and a LOT more expensive. I hear that there are third party alternatives that are much cheaper (but still good) that will fit an FD mount. Does anyone know about any of these third party lenses for FD mount, particularly ultra wide (eg. 17mm or lower) ones?
In FD mount, your pickings are going to be extremely slim if you're not looking at Canon first-party lenses. You'll be mostly limited to Vivitars, Tokinas, and maybe Sigmas, and this means that most of your options will suck.
Basically, you will find nothing vintage that is both cheap and good at the ultrawide end. Remember, there WAS no APS-C. Everything was 35mm film, so 17mm, which is only moderately wide on APS-C digital and midrange on M4/3, is RIDICULOUSLY wide. Thus, expensive, specialty, and rare.
Take a look at this article. Alan's a vintage lens geek, and his opinions are usually pretty good. It gives you some lower-cost options from Tokina and Vivitar; nothing wider than 17mm, though.
You should also note his advice on which adapters may cause the back element of these ultrawides to hit your focal reducer lens; that's a big consideration, and will ruin your day in a hurry.