You're looking for the AICS bottom metal by CDi Precision
. I don't think it's something you do to save money, considering that magazines run $75-80 to begin with, but I'm told they're better-quality and more easily available than FN stock ones.
As another alternative, I hear that Williams Firearms Company can convert your DBM to a floorplate model
, but I'm not 100% sure how it works. I've been thinking about talking to them about it, but nothing definite yet.