Welcome to the wild and wacky world of dressing up like random anime characters!
You’ll know you’ve become part of our world when you can discuss your son “crossplaying” without raising an eyebrow, and you have matter-of-fact discussions about which costumes make it easier to fake various parts of anatomy that regular (non-animated) humans don’t come with.
So, for your first question. Do cosplayers sell their used costumes? Nope. Most cosplayers are rabid creators, and are very proud of their costumes, which they intend to keep on wearing to cons until the sun burns out. They would also rather die than have anyone see that those lovely-on-the-outside costumes are often held together with duct tape, spit, baling wire, and a few well-placed prayers to specific deities.
You don’t want a cosplayer. Instead, you want a COSTUMER.
You are going to have your best luck finding a costumer at an actual convention. (Call them cons) If you’re in the SouthEast USA, Anime Weekend Atlanta is a really family-friendly, kid-friendly place to start. It was the first con I took my own little brother to, and he had a blast. There are others, both small and large. I would suggest a small one for his first time, especially if there’s a local cosplay group or anime club that hosts one nearby.
If you go to a con, I highly suggest picking an anime to dress up from that involves an easy costume. Look for something where the characters wear modern clothes, or easy-to-fake historical outfits, or a series set in space where they wear coveralls. Look through the anime that he likes, and pick a character or two that he somewhat resembles, and print out a color picture of each. Bonus points if you can find a picture of their BACK as well. Remember to include any props that they have (sword, weird gun (look at nerf toys) or tools/accessories). Then, just go poking around looking at clothes and things everywhere. I would suggest thrift stores, hardware stores, and “clearance stores” like Ross or TJMaxx.
Look for wigs and accessories at all the upcoming Halloween supply stores that are popping up like fungus now, but try not to buy your major clothing bits there - it will be too expensive and bad quality. Look at thrift stores and clearance stores. Remember that regardless of what the actual costume includes, wearing comfortable shoes is a requirement.
Then, go to the con, have him wear his first cosplay ever, and take a camera with you.
When you see a cosplayer wearing a lovely costume, you want to do two things. First, compliment their work. (Always assume that they made it themselves.) Then ask if they take jobs for people. If they hand you a business card or phone number or web-address, you’re in business. Sadly, most cosplayers don’t have the time, funding, or equipment to make entire costumes for other people (they’re usually kept too busy making costumes for themselves - see above references to baling wire and spit.)
What you’ll most often find are people who do specific things - some people work with wigs, others make accessories (belts or harness things or shoe-disguises) and others will make articles of clothing, but not an entire costume.
You’ll also find vendors and salespeople who sell clothing and costume pieces, usually iconic and hard-to-make items. It’s very rare that you’ll find someone selling “costumes in a bag” like you find at Halloween stores, where everything from shoes to accessories to outfit to wig are all together. It’s also rare that you’ll find anyone at all willing to undertake that sort of job for you.
It’s partly because of the limitations people have on their time, and partly because the cosplaying scene has a really strong DIY vibe. It’s considered bad form or not quite cosplaying if you buy a costume entirely “off the rack.” Besides, you feel more proud of things if you put the effort into either hunting down bits and pieces to assemble into a costume (which lots of people do - not every cosplayer is a good seamstress/tailor) or make it from scratch from expensive fabric.
If you have more questions, please ask! It’s really a very fun hobby to get into, and you may soon find yourself buying your son a sewing machine! 
