Charles,
Calbiochem has it, rabbit anti-human factor XIIIa that is,  the price has
increased to 160 dollars or so for 1 ml. I remember buying my first ml for
35 dollars in 1997 or so. Wrote a lot of papers using that first ml. I used
the Calbiochem product at 1:400 in PBS with 1% BSA after 20-30 minute
trypsin (Sigma tablets) digestion  at 37 deg C for FFPE tissues. Biogenex
sells a prediluted and concentrate polyclonal as well (concentrate is best
at 1:30 to 1:60 in PBS, no carrier protein necessary they add it already-
they recommend citrate HIER). Biocare Medical also lists FXIIIa and
Novacastra has one too.
Hope this helps.
Jeff Silverman HT HTL QIHC
Southside Hospital
Bay Shore NY USA