Hello Stephen,
ATM modules for LAN switch 5505 should be LANE LAN emulation modules they use ATM but for LAN emulation so I don't think you can use them as a generic ATM switch.
I've read several people have used two routers with SHDSL module connected in back to back to do some practice with ATM.
Or also you can try to get an ATM switch of other vendor for example FORE systems 1000 can be good.
I don't know if you can find one at a cheap price. Of course there is the added burden to learn its CLI.
Hope to help
Giuseppe