One of the big reveals in Chapter 24 was the fact that the RPG that the Mallers were using came from Burt's gun store, Locked & Loaded. Apparently Burt had been selling illegal weapons on the black market! The RPG and its ammo were in his front safe because the buyer was scheduled to come in to get it.
I think this answers the question of how, and why the Mallers hit Burt's store so quickly at the start of the outbreak. My theory is that the buyer for the RPG was someone associated with the Mallers (probably not Scratch since there's no indication that she or Burt recognized one-another). The deal could have been brokered through a middle-man, or someone on the Maller team that we haven't seen. I think that Scratch always planned to steal the RPG and rob the store, and the theft just happened to take place on the day of the outbreak. That explains how it was organized so quickly, and how Scratch just happened to have a safe cracker with her for the raid.
An RPG, a load of guns with ammo, and, maybe, a supply of C4 explosives would be very handy if you wanted to break someone out of prison.