Here is a link to GunMag Warehouse. They have them in stock. I just bought some for my rifle. They also have the 10 rounders in the 25 round magazine length I guess for folks in restricted states that want to have a full size mag look. I'm in Texas and can have the 25 rounders but like others have said, I only load 10 at a time and don't always want that long mag hanging down when I shoot.
I don't post much here but I do as Greg G on other sites such as Jan Still's Luger site.

Smith & Wesson S&W M&P 15-22 22 Long Rifle 10-Round Short Polymer Factory Magazine