If the plate's gone, it will be harder.

They may be able to work from dimensions. What I'd recommend is this:

#1 - measure and record the height and diameter of the filter.
#2 - photo the filter and the plate and email them to poolforum AT gmail DOT com, and I'll post them in this thread.
#3 - copy the link to this thread, open another browser window with your email client, and copy the link URL for this thread into that blank email. Add the dimensions.
#4 - call A1, ask for the reps email address and send them the email. Ask them to take a look.
#5 - Proceed from there.

Ben