Professional attire?

I’m 5’1” and 90ish lbs, 23.5” waist. I work in a fairly casual office and wear the same pair of jeans pretty much everyday.

I have an upcoming work trip where I’ve been told equally to dress more professionally and to not overdress. I assume this means slacks and a nice blouse — but what? Where? Most of my clothes are too big, it’s my first foray into corporate America, and I’ve lost MORE weight from the stress/adjustment period. My wardrobe hasn’t been updated since college and I really just don’t know where to shop.

What do I wear to make myself look polished/older/essentially not like a kid playing office dress-up? I am open to literally any suggestions (brands, styles — I’d be happy with a Pinterest board at this point). All I have is a pair of wide-leg black slacks that I’ve been told look “comfortable” 🤦‍♀️