what exactly are you asking? I don't understand the quetion. Are you trying to fill in the fret slots after converting to fretless, or the other way around?
You cant buy fret slots. A fret slot is what the fret is put into. So basically if you pull the frets out of your bass then the remaining holes are Fret slots.
Some people use a wood putty, which is a wood/epoxy compound. Others glue in veneer strips of wood to fit the slot. SOme put straight epoxy in there. There are quite a few ways to fill the slots.
plastic works ok. Yu can get some from the little packages you get with acoustic guitars. They are usually used as shims for the bridge piece. Or, epoxy, which is harder wo work with, but more durable.
Elmers Wood Filler, - after filling the slots, wait 5 minutes, clean exces with a clothe and ready (wait more if you are leaving it that way) if not - sand with nº 400 sand paper - then sand with nº 1500 sand paper and apply wood sealler.