Do you use one and do they really keep your pins sharp?

I have one, and it's great. It takes the little burrs off pins and needles, and also the gunk that accumulates when using a fused interfacinf.

Where do you get the emery pincushion? What do they look like?

The little strawberry that used to be attached to a tomato pincushion was always filled with emery powder. Otherwise, you can usually get emery powder fro lace equipment suppliers. If you use it as is, it's quite hard to push the pins in, I used to mix it without about half/half of sawdust. That kept the grains of emery powder slightly apart, so that it was easier to push the pins in. Don't make it too large, emery is very heavy.

Thank you :) I have one of those tomato pincushions and just put a needle through the strawberry. Never knew it was different.

My quilt guild made Strawberry Pincushions for one of our programs several years ago.
Made them out of wool. Mine is pinkish red, then I put french knots on it to look like a strawberry. Put a green leaf on top of the opening after it was gathered up to look like a strawberry. We filled them with play sand. Made several for gifts. They work great.

Every pin cushion I have that has sand in it, the sand will find a way out through the pins stuck in it so don't have it around your machine. A few sand grains will ruin a motor or gears. I now use rice for weight and wool batting pulled into fluff for the filler.