Boy's/men's ballroom clothing is generally basic black. True stretch, body molding Latin pants and the true Latin shirts that snap at the crotch can be expensive as they are tailor made. At 9 your young man is still growing. It might be less expensive to shop for black slacks and a black shirt for him to dance in. If you wish you can embellish the shirt with crystal stones to punch it up. I would save a little money and order percosa stones. They have a lot of sparkle on the dance floor, but don't cost as much as the Swarvoski stones. I like ordering my stones from The Rhinestone Guy, he sells it all. http://www.rhinestoneguy.com/index.html
You might try E-bay typing in Men's Ballroom. Be sure to try Used Men's Ballroom too.
Andrew Smart's, World Promotion Competitions - one day events are less on strict rules and more on fun. I've seen lots of little girls in sequined costumes. And boy does Andrew like to let everone know he WILL play AC/DC music in the Latin heats. His goal is to encourage more children to dance so All children's heats are FREE. They still have to pay to get into the ballroom as well as for the lunch & dinner.
This does not help answer your question but if anyone can help her, I have a 9 year old grand-daugher who would also be interested. A partner would be nice also if anyone has young boys looking for a partner.