There aren't any simulators per se, but you can usually find something like a Fore Systems / Marconi ATM switch (like an LE155 or ASX 200BX) on eBay for a pretty reasonable prices (less than an ISDN simulator).
They are pretty standard ATM, can handle all the LANE server / services functions (LECS, LES, BUS, LEC) ... can be set for NNI / UNI, do QOS, ...everything.
You can also set up the ATM UNI on your 5500s for RFC 1483 bridging (basically a switch-to-switch PVC) without any additional resources (I believe ...).
Good Luck
Scott