Never. Abandon all hope!
Not really, but in all seriousness, it seems like every few months or so there is a drop somewhere, but when some does manage to drop on the bigger sites it disappears like a live chicken tossed into a hyena pit.
Keep your eyes on various smaller local B&M tobacco shop websites and you may see some pembroke or margate show up on occasion you can scoop up. Otherwise make friends with B&M shops that have an Arango hookup or suffer the ebay/consignment tin shop prices. Another option might be to trade with forum members once you have enough posts to do so within forum rules. Some folks here have quite the stash and are often willing to help others find the blends they want to try.
In the meantime, ask forum members for blends they find to be similar, or at least similar in quality, that are readily available. There is a lot of good shit out there that can be on your doorstep by friday for much less cash.