I tend to carry a stock of supplies, so have custards, syrups, fluff, frosting, foam and lots of gunge - probably other things too. Shopping needs tend to be dictated by things like Candy's Wamstructions for me,
Living in the UK I'd have to put Cool Whip/Stabilized Whip Cream on the list so that I could dream that somebody would actually acquire some. I'd probably then add the more realistic items of custard, food colouring and Cake batter.
Shaving foam, paper plates and/or foil pie tins, food colouring or poster paint to colour the pies, custard, buckets and bowls for mixing, and then binbags and cloths to clean up afterwards