my son has about 50-60 soccer jerseys (international, european, south american, MLS) bc that's literally all he'll wear as a shirt so i've had to find the better websites to get deals. Pretty much have gone through these and they're all reliable. I just go to the clearance section and pick out a bunch of sale shit since he's not picky about anything outside of the EPL (man city) or MLS (austin).
https://www.soccerpro.com/
https://www.soccer.com/shop/sale
https://www.worldsoccershop.com/
https://www.prodirectsport.us/l/kids/departments-clothing/
found a couple of random good ones for cheap here
https://www.shoebacca.com/apparel/search/age/youth/style/jersey
https://www.lids.com/soccer-kids/o-1434+ga-58+z-99009577-1211520045?_ref=p-GALP:m-SIDE_NAV
this one seems sketch and took a while, but i did get jerseys after about 4-5 weeks. Order a size bigger than normal.
https://hamptonathletics.com/en-us/collections/frontpage?page=1
LMK if you find any good other sites.