If your priority is breadth of extended sizing + genuinely useful fit options, I’d put these vendors at the top of the list:
1. Lands’ End — best overall
This would be my first vendor to investigate if you need a large size range rather than simply a few “plus” products. Lands’ End has a particularly strong school-uniform assortment—polos, oxfords, sweaters, pants, skirts, etc.—and offers regular and husky sizing on many children's uniform pieces.
For example, its school-uniform interlock polo is specifically offered in Kids 2–20 with regular/husky options.
Why I like it: It combines uniform-specific products with a much broader everyday children's clothing assortment, so you're less likely to have to use one supplier for uniforms and another for casual shirts.
2. French Toast — best dedicated uniform specialist
If you're primarily buying school uniforms, French Toast is one of the strongest choices. Its range includes polos, oxfords, pants, shorts, skirts and other uniform staples, with husky sizing extending into the teen sizes on selected items.
For example, its short-sleeve pique polo is offered in sizes 4–20 Husky, while the long-sleeve version is available in 10–20 Husky.
Best use: schools or families that want a consistent uniform wardrobe without having to piece together different brands.
3. Old Navy — excellent for inexpensive everyday shirts
Old Navy is worth considering if "everyday shirts" is just as important as uniforms. Its uniform polos come in extended/husky variants, and the broader children's assortment gives you considerably more casual choices than a dedicated uniform company.
Current listings include both jersey and pique uniform polos, including husky variants.
Best use: mixing school-compliant basics with inexpensive everyday clothing.
4. The Children’s Place — particularly good for plus sizing
The Children's Place has some useful plus-size uniform basics. For instance, its uniform pique polo is currently offered as a plus-size item.
I'd rank it below 
Lands' End and French Toast for overall depth, but it's a good secondary source when you need a particular fit at a lower price.
5. Walmart / Wonder Nation — strongest budget option
For families needing multiple shirts, Walmart can be very competitive. Wonder Nation has school polos in husky sizing, including inexpensive 2-packs.
The tradeoff is that the assortment and consistency of extended fits can vary more than at Lands' End or French Toast.
My shortlist
If I were building a purchasing list, I'd start with:
- Lands' End — widest combination of extended sizing, uniform pieces and everyday clothing.
- French Toast — strongest dedicated school-uniform assortment.
- Old Navy — best combination of uniform basics and casual/everyday shirts.
- The Children's Place — good plus-size/value alternative.
- Walmart/Wonder Nation — best for low-cost bulk purchasing.
One important distinction: "extended size" doesn't necessarily mean an inclusive fit. For a child who needs extra room through the chest, waist, stomach, or arms, I'd specifically compare husky/plus cuts, garment measurements, stretch content, and whether the shirt is relaxed vs. slim cut, rather than relying on the numerical size alone.
If you tell me the age/grade, approximate size (e.g. 14 Husky, 16 Plus, 18 Husky), and whether you need boys', girls', or gender-neutral shirts, I can narrow this to the 5–10 vendors with the deepest actual size ranges and compare their maximum sizes and fits.
