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.