Officially Sigma are no more, but Thomann - a german mail order company - seems to have them for sale.
Go to www.thomann.de and search for sigma guitars - they have more than 10, all strikingly similar to Martin models, in naming and spec, many in stock, the others available in 2 to 5 days.
Has production restarted?
I sent an email to Martin to ask but they didn't answer (yet)