How do you support your husbands/S.O. crossdressing while also staying true to yourself?

You need to discuss and determine what's acceptable, and what's not. Maybe its ok around the house, but not out in public. Or maybe if he wants to go out in public, its ok but not with you.

Maybe let him wear some of your older things that you don't really wear anymore, if they fit? Definitely help him buy his own underwear, so he doesn't stretch yours out. Maybe go shopping/thrifting with him?

You don't mention if you have kids or not. It can be a bit more open if you don't have kids at home.