I've read on here that a common blank for streetwear brands is the Anvil 1301 or 1701. But I don't know about the specific brands you mentioned. Most retail brands have their garments custom made to their specifications. They probably have several overseas factories that handle their production, they do not use common domestic suppliers. You can check the RN Number that is on the neck label, but it will probably just give you the brand info, not the original manufacturing facility info.
If you are doing high volume, you can contact an overseas factory or a domestic supplier and ask them about private label programs. You can either spec out the shirt or just send them the sample and they will replicate it.
If you are doing low volume, you are better off using a domestic supplier and buying their off-the-shelf blanks. You may not find an exact match, but you should find some quality blanks to get started. You can always upgrade to custom specs as your volume and brand grows.
What is it about these brand's shirts that you like? That will help in recommending a domestic supplier.
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