My DD does Deca. She got a suit at Express that was a little pricey. She got one at Macy's also the BarIII brand.

If you just need pants/tops that section of macy's would be good. Tysons had a decent amount of pants and tops. DD needed jackets which were a little harder.

Could also check a consignment or nice thrift shop.

Actually the teen interns I have seen do generally have a few basics from Ann Taylor, loft, br, etc. Just get one or 2 sheath dresses, a blazer, a cardigan, black pencil skirt and maybe black and cream pants. She probably has a few tops that can work, especially if she throws the blazer over. H&M generally has some dresses that will work. I’d stick to classic enough that she can wear them next year too. Simple wrap dresses, etc

Mine actually does just borrow my work clothes, including stuff like Ann Taylor. Simple black pants, simple cotton or silk shirt, no belly showing. It’s not like she’s hanging out with her friends so she’s not really trying to look cool. She also has a simple black skirt that is banana republic I think, and a BR blazer and I think a cardigan from gap.
I think dresses are harder to find something that looks age appropriate but also office appropriate. She has one we found at gap she likes.

Express
Ardene
J crew
H&M
Zara

I like the suggestions above. also, Uniqlo would be good.

old navy pixie pants.

She should be able to find something appropriate at H&M.

DD has an internship this summer that requires her to wear business casual clothes. I'm at a loss where to find such clothes for a teen. She's not going to wear anything from say, Ann Taylor. Any suggestions?