MrsPurple - you're too nice!
Hi Tadpole. You've already had some good advice here, playsand is widely available and cheap, no need for any expensive marine sand from the lfs.
I've previously tried silica sand and found that ocellatus shred their fins on it, it's too sharp. Also tried a mix of silica sand, crushed coral and aquarium gravel - multis couldn't dig in it and only large ocellatus could move the gravel. I've found that playsand alone or a mix of black aquarium sand and playsand gives the shellies just what they want - the ability to dig, bury their shells, dig them out again, move them around, rebury them, unbury them...etc you get the idea?
It shouldn't really need saying that you need shells too, these are often more difficult to come by but just about any type of shell should do so long as the fish can get in and out (some marine shells are not smooth inside and the fish can get caught.) With all shellies, you need more shells than fish. They breed readily so be prepared to add more as the population expands!
You need to do some research on your shellies - if you go for multis or similis you need to place the shells in groups, if you go for ocellatus or meleagris you need to spread them around something like 3-4" apart. Multies, the male in the main, will dig to the bottom of the tank and build ramparts around his territory. Ocellatus will dug under their chosen shell to bury it leaving only the opening exposed. Males and females will cover unused shells in their territory.
With multis I would recommend that you put some flat stone or slate on the bottom of the tank and the sand substrate over. This way when the multies dig to the bottom they won't reveal the glass on the bottom.