If you just want the plastic bottom metal you can get it from E. Arthur Brown.

https://www.eabco.net/Savage-Long-Ac...l_p_16056.html

This will use the Axis style magazines with the plastic bottom and integrated plastic retention clip.

You will probably need to order the Axis magazine bottom cap from Savage to use with your existing mag box/follower as I'm not aware of any retailers carrying them (maybe try Gun Shack?). The other option is to just purchase a complete Axis magazine.