So many things going on at once.

Did you wear a kilt to your business meeting? Was this not enough to show your mother you can dress yourself appropriately?

You could call the police, but you would have to be willing to prosecute the theft.

Perhaps a church leader could explain the concept of theft to both your mother and mother-in-law.

You're a grown man, married, with children, but your mother took the kilts, not the kids, because she was worried about you? Seems she has the wrong priorities. I don't know how old your kids are, but her theft is the wrong message to give them.

I like the letter idea, though it takes too long; you need to hand-deliver it. Take a deep breath and stay there while she reads it (don't get into a discussion with her about it). And do not leave until you have the kilts.

She should also apologize for violating you, your marriage, your family, and your house.