I see a number of items on this forum about where to find holsters for SDs (and Sigmas).
One inexpensive but very nice leather holster (with thumb snap), made by a small manufacturer in North Carolina, that fits the SD is for sale on eBay for $39.95. This strikes me as a good bargain. I have two of them, one brown and black, and they fit my M&Ps and my SDs.
The seller is coye57.
(Full disclosure: I have no connection with coye57 or anything to do with the company. I just like the product and find its low price to be noteworthy).