Inositol Triphosphate (IP3) measurement in cells

[S.E.]

We were using PerkinElmer's Inositol 1,4,5 Triphosphate RIA Kit (Cat #
NEK-064) to measure IP3 extracted from tissues.  We now want to
perform the same experiment on cells.

The problem is that PerkinElmer's kit is backordered indefinitely. 
Now what?  Does anybody have a protocol that describes how to do this
the old-fashioned way?  Is there another manufacturer who has a
similar kit?

Any help, advice, input etc.  is very much appreciated.

Thanks, 
Stephanie