Is there another way to do it, besides with the IG with all gems, or Batbelt with 3+ resources.

Can always use DKR017 Harvey Dent

I BELIEVE IN HARVEY DENT: Friendly characters adjacent to Harvey Dent at the beginning of the turn can use Willpower until the end of the turn.

also in golden age theres a battlefield condition card that gives everyone on the map (both teams) willpower, i believe it's called legendary day. another pseudo method is to make someone a herald of galactus

now there is also Thomas Wayne. Love him!

I believe the Asgardian ATA gives willpower to those using the Keyword on their first click only

Sure, super easy way. A little white marker. You just put a line through the Defense Shield on your figure. Goes from to this like magic.

Oh, also, not exactly indomitable but Edward Nigma and Warbound allow you to give tokens to other figures which is often better as it allows the figure to act every turn.