Mike,
I have had the same experience; the IPMS Canadian Western Regional has 5-600 models entered per year, and a pretty impressive Vendors area however there's very little WWI stuff to be had. Entries are about on par with the totals, 10-12 WWI Aircraft per year. We are a very small niche of the Modelling community. Last year I was able to attend the Ottawa "CapCon" version, and found it was near identical in WWI entries and vendor's opportunities. If I were selling I'd do it on line here, or on EBay etc.
Cheers,
Lance