Symphonic Band Attire:
Boys: Black dress pants (not provided) and Black long sleeve button-down collared dress shirts. Black belt and black dress shoes…and of course LONG BLACK SOCKS!

Girls: All black dress clothes. Black dress pants or black long skirt/dress down to the ankles and BLACK dress shirt or blouse. Black belt (if worn) and black dress shoes…and of course LONG BLACK SOCKS (if worn)! Basically, concert black formal dress clothing.
—No sandals/flip-flops
​—No short skirts
—No black jeans
—No leggings or yoga pants
—Please remember this is a band concert and not a fashion show. Please dress appropriately.

Some affordable options:
https://www.concertattire.com/Laydown_Collar_Dress_Shirts_Style_M03_p/m03.htm
https://www.demoulin.com/ladies-laydown-collar-blouse-1
https://www.demoulin.com/blouse-with-soft-v-neckline-and-3-4-sleeves
https://www.demoulin.com/formal-attire/women/blouses
https://www.demoulin.com/formal-attire/men/shirts

Men- Mr. Shoppe probably has several options for black dress shirts for men

Examples