Does anyone know a good place online to buy 16+ round mags, blued or polymer, something inexpensive.

Check auctionarms.com. I bought some mags for my .40 from pawnshop in your state alot cheaper than from S&W. Hope that helps.

I saw in a Midway catalog that someone is now making aftermarket magazines for the Sigma now.  IMO, they aren't worth it.  Just look for some used ones on the forums, or post a WTB ad on the S&W Forum.  They are usually reasonably priced.  The worst that can happen with used mags (since they are steel and not plastic POS mags like another brand I won't mention) is that you need to replace the follower if it's a little worn and not locking the slide back on an empty mag.  I have a lot of mags for my Sigmas, many that I've bought used, and never had to replace a mag spring yet.  Followers are cheap right from the factory.

If you look around, you should be able to find used mags for about $20 each.

I agree you can check out the S&W forum for good used factory mags.  I have a number of mags for my 40VE and my 6946 and most have been used with no issue on mag springs.  Price range was 10 to 20 per mag.

So, look around for them and you will find some... :)