Near positive that Remington never made anything other than the 5rd for the 40-XB repeaters. Have never seen one for sale and heard prices run as high as $200ish which is unbeleivable.
Now with all that said I know who has or at least had some this spring for sale. I am suspecting you are wanting to find a nice used one from a private individual but think anyone who still owns a rifle won't part with a mag. Now without further ramblings contact this nice gent.......
Wendall @ Out Back Gun Parts, Inc.
2110 McLean Ave.
New Albany, IN 47150
He is only open Tues-Fri from 10:00 to 5:00PM. He doesn't have a website. IIRC he told me he bought all or some of Remington's inventory on the rimfires and about a dozen new mags from an estate several yrs back. Super nice guy to do business with.