AFAIK the last Sig P210 magazines were being sold by CDNN. I bought two of them from CDNN in 2002 at $80 a piece. The P210 was dropped from production by SAN, the successor to SIG in Switzerland in late 2004. It has been at least two years since CDNN dropped the 210 mag from their website and quarterly catalong. Except for private sales, I do not know of any supplier offering 210 mags at this time. I wish Meg-Gar would tool up for a production run. Just think of a Mec-Gar 210 mag that sells for $25-30. I'd buy a few.
I've heard the story about Walther P38 magazines being used in the P210. I own a commerical grade P38 made in 1984 which puts it as one of the last ones made. I have four factory mags for it. Two blued that came with the P38 and two parkerized mags that were made for the West German Army's P1. The P38 mags will go into the P210's mag well, but they are a hair too long for the P210's heel mag latch to lock on to. I suppose that one could do a bit of handfitting on the P38's floorplate and make it work satisfactorly. I have four P210 mags, so I have never bothered to give it a try. If I ever find a cheap and expendable P38 mag, I will certainly see what can done to make it work.