The most purple I've seen is probably the "Purple X" line from Simon McCleave in Belfast, Ireland, though recent photos have not impressed me as much. Orchids and some Amethysts (Lava Lavender) have had the best purple color in my opinion, but only certain lines. A lot of breeders (especially the majority of hobbyists) only seem to focus on the gene mutations and frequently ignore polygenetic traits, either because line-bred animals with better coloration/patterning are often more expensive or because it just never occurs to them to do so. Depending on the size of the expo, you may or may not find what you're looking for. I went to one of the largest shows in the US last fall and don't recall seeing very many lavender mutants...perhaps a few on Walter Smith's table?
You might get lucky, though. Keep a sharp eye out. If you don't find something perfect, don't feel like you have to buy something right then and there. Plenty of breeders will have hatchlings available starting in June/July.