I look at World of BPs and go through the morph list to see what I want to make eventually and what I will need to make those. I compare what I need to what I have and go from there: I don't think you can go wrong with picking up a 2-3 gene male and maybe a single gene female at the show depending on prices.

On a note: I got two sick snakes from the same vendor at the last expo I went to that involved some expensive vet care. Make sure you buy from good sources at the expo (maybe research the vendor list on the BOI beforehand) and closely inspect any snakes you'll buy. Know that mites or RIs or other illnesses can be risk with expo animals. I think I just got unlucky with my two since I've gotten many others that turned out just fine!