I can think of a couple cards that could accomplish that

Wonder would give all creatures flying when it's in your graveyard

You could also use Coat of Arms + Abzan falconer to give your army of squirrels a bunch of +1/+1 counters since they're all the same type. Falconer means that everything with a +1/+1 counter will have flying. Both of these aren't green though, so you'd need a hybrid deck or some kind.

Eldrazi Monument and Akroma's memorial would also give flying, and are colorless to boot. Eldrazi monument makes you sacrifice a creature each turn though (not a huge deal with how many squirrels you can get). Akroma's memorial doesn't have any downsides, but is 7 mana which is decently expensive.

I'm sure there are other ways but this is what I can think of.

all thanks to the squirrel totem of course, not the most effective but for sure amusing (inscryption reference)