I can't imagine why the .50AE RCBS mold would be on backorder for three months! Usually RCBS is very good about keeping their distributors stocked. They pump out custom jobs quicker than three months in most cases. Yes, indeed the mold does exist, and from what I understand it is a dandy for the AE.
I'm not sure who makes a bullet right now for the .50AE in cast on a commercial basis. We should have two bullets available in about a month in our lineup. Both of them for the .50AE specifically. I'll not quote weights until I get the molds, and actually cast them with our alloy, as it is different from most commercial alloys, having an antimony content of only 3%, it casts heavier than most (giving greater sectional density and being less brittle)
However, until then I'm not sure where to send you for bullets, but you might try this link for the mold:
Huntington's Die Specialties
They are a little more expensive than others, but being in Oroville, CA, where RCBS is located, they have direct, first access to everything that they make. If they don't have it in stock, you'll never know it, because they'll simply get it ASAP from RCBS!
Hope this helps a bit!
God Bless,
Marshall