ORWO just ran some kind of a promotion so that's likely why it is sold out. On the other hand, the good news is that it is sold out. As long as people keep buying it they hopefully will keep making it! I have plenty of UN-54 right now so I didn't get in on the deal.
Can't say I've used Plus-X recently, if ever, but I have also heard many comment that UN-54 is a good replacement for Plus-X. You can sign up on ORWOs website for promotional emails. If you are concerned about spam, it isn't a problem because they don't send very many.