Are you referring to that for setting custom WB? If so, a cheap white styrofoam coffee cup works well for me. Or a Pringles plastic lid... far cheaper than Expodisc.
Then again, do know your camera's idiosyncracies.
Just look for any white surface. I've tried setting the WB using my white T-shirt and it turns out quite alright. There are some photographers who will carry a piece of white cardboard with them for this purpose too and some will use normal A4 sized paper for photocopying.
If you really want a Grey Card, can get from Cathay Photo at around $20 IIRC.