If you want information concerning how citizens can prepare for a CBERN weapon, you can start by visiting the FEMA CERT site or the Fire Institute web pages.
"Gas masks" are for limited use and need to be fitted to each person to be effective. A mask in itself may be inadequate if you don't have a full biological suit with scba. It depends on the agent. Such suits are very expensive and they are made only for adults. In general, the best option is to learn to shelter in place and wait it out for the agent to be dispersed.
Thanks!