I got a smock from k-mart but i have also seen them in woolies in the baby isle with the bottles, bibs, cups etc etc. Maybe try woolies if you have one near you. Were abouts are you located MummaB?
I let me girls go for there lives and then just hose the paving down after they have finished! I must say though i actually get out there and paint as well and make a mess too part of the fun really - i act like a kid too Lol!

HTH
jen