Electrets cannot be be recharged as they won't ever loose the charge in the first place. Any imbalance has to have an external cause such as broken cable, parasitic charge on the film, damaged driver frame etc. The charge won't "wear out" in the foreseeable future.
Never seen electret film for sale and I doubt you could buy just a small roll of it. It's interesting how it is made though, not something you can do at home.