I would consider Singh-Ray to be on a par with Lee, they have filters to fit both Cokin P and Lee adaptors.
As to availability, it probably depends on the market. In the UK, I had to wait 9 months for a grad set last year, but someone I know bought a set a few weeks ago and they arrived in less than a week, so supply issues have eased. Bear in mind though, that Lee tend to produce to order, rather than keep a stock, so there will be supply problems of certain items from time to time. Some suppliers will however keep their own stock.
Ah ok because bh and adorama see to not have much of an option, do you by chance know of any other sites that have a better selection? if I google sites none have any.