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Where do you get them?

Turns out the polarity doesn't matter.

Dremel also has a small sanding drum that is good for edges such as the edges of raised letters.

For flat carved surfaces such as your project (or behind raised lettering) I use one of these Proxxon pen sanders: https://www.proxxon.com/us/micromot/28594.php
It requires a 12v power supply, but...