Something I strongly suggest if you need something specific is to remember that the IWM folks are fantastic, but they aren't Battletech players. Telling them you need an Ultra AC/20 for a Blitzkrieg may or may not get what you want. So what I do is two things:
1) Look up the ID number on their webstore for the miniature, even if spare parts for it aren't listed. They don't know it as a name, they know it as an ID number- so help them out a bit on that. Do some research too if you can- if you need a right arm, great, but if that arm comes on a sprue with other parts it helps to let them know what they're looking for.
2) Grab a photo of the miniature if you can, and use something like MS Paint to circle what you need. If all else fails at least showing them what the part looks like helps a ton. YOU know what it looks like, but just seeing 'oh, THAT part' is huge for them.