A quick look at the Intellectual Property Office website shows that 'Osiris' is a trade mark owned by Specsavers. (i.e. it's an 'own brand' product offered by that company). So there's absolutely no point in looking anywhere else, since only Specsavers stock the 'Osiris' range.
Looking at the Specsavers website shows that they've now discontinued the 'B20' style. Further, it also shows that all of their stores are individually owned (i.e. it's a franchise business) so individual store bosses probably won't be able to access any old stocks that might be held at other Specsavers stores.
Therefore your best bet is probably to contact Specsavers head office directly, as they might still have some B20 frames in their central stock or they might be able to email all their franchise holders to see if there are still any left in individual stores:
https://www.specsavers.co.uk/help/contact-us?popup=&wssl=1