What to substitute a lost pedal dust cap with?
By the time you have driven around town looking for a suitable sized dust cap, new Pedals might even work out cheaper.
One option could be use the pedal itself as a mold to make one from a silicon sealer.
Local hardware store will have a range of plastic end caps for things like curtain rods chair legs along with other and general hardware purposes.  You might find a suitable sized item there. Its the kind of thing an LBS might have in a box of bits.