This is a 25 round 9mm Sten mag with an AR mag block attached permanently to the mag. I think that I'll be in the market for more in a week or two. Does anyone know who sells them?

If I recall right, that is the system that Olympic Arms uses.  I would start there..

Thanks.

The rifle has an Olympic 9mm upper on a Essential Arms mil-spec lower.

Yeah, good luck trying to get something from Oly Arms. I had an upper from them years ago that used a different block and the older Simutions mags (heavy duty plastic) and had some problems with a couple mags. Called them and told them my issue. They said they would put the request in and ship out when ready. 6 Months later I got the replacements. Hopefully Oly Arms is running better at individual customer service.

Also might want to check Brownells, they carried them at one time.

I tried to find a thumb saver for them,  reg. Sten saver wouldn't work for me.

I may just get a regular mag block that fits in the magwell and uses standard Sten mags.