Jenny at sew-classic.com has a couple of different kinds for slants.
I found a vintage 301 darning foot on eBay, but honestly, I prefer the one with the square lucite "foot" to the very expensive original foot. :)
The visibility is actually very good - but you might have to spend half an hour quilting with it to get used to it.
I think you have to remove the lowest thread - or maybe it was the thread cutter - to get any but the original vintage foot to fit on some 301's. Mine didn't seem to have this problem, but I know some have.