The Olympus eSif lists the slide-on hood for the 55/1.2 as US part# 108-233, while the hood for the 50/1.4 (and 50/1.8, 35/2.8, etc.) is part# 108-230. The difference is that the 108-233 can be reversed for storage on the 50/1.2. I would guess that the 108-230 is just a tiny bit smaller in diameter, making reversal storage very difficult if not impossible. In fact, I find the 108-230 difficult to store on my late production 50/1.8 miJ. My s&le of 50/1.8 miJ is ever so slightly larger in diameter than my other 50/1.8s and my 50/1.4. The difference seems to be in the focusing ring rubber.
Well, that's a VERY good question. It's going to be fairly difficult. oBoy is an obvious possibility, but I would also check KEH and other sellers/shops with an online presence. If you belong to the Olympus list (