Here’s an example of what a supply chain can look like for fabric:
Grower/Raw Yarn Producer > Fabric Mill > Wholesale Dealer > Sourcing Agent > Manufacturer > Reseller > Brand > You
The further you are down that supply chain, the harder it becomes to ensure each step was done right. You as the customer are in the toughest spot.
Many of the best producers in Canada insist on full service manufacturing or they won’t work with you. That means they source and control your fabric, patterns and materials sourcing. It’s become so tough to make money manufacturing here, many factories refuse to do just do the cut/sew production portion of the supply chain - it's just not worth it financially.
As a small brand, you start off ordering white label products (blanks) because you can't yet make 300-500 units per colour/style which is a general area you need to be in to make manufacturing here feasible - or for the manufacturers to consider working with you. In both these cases you know the manufacturer or reseller onwards in the supply chain.
If you care, you ask to see where they got all the raw materials but they are hesitant to share because they fear you cutting them out and going direct to source to cut your cost. In most cases it’s an act of self preservation, not in order to hide unethical sourcing - but there's no way to differentiate that.
We once had a Canadian production partner refuse to even provide invoices for purchases he made on our behalf because he feared us cutting him out - even with a contract and non-disclosure in place. He first redacted invoices and then refused to provide them anymore. He also knowingly mislabelled a ton of shirts to make extra money and tried to ship them out without us knowing, so poor ethics exist here too. We fired him instead of cutting him out.
Many times the manufacturers may not even know the source themselves since they are buying from a dealer. If asked, the dealer isn't excited about sharing his sources for fear of losing his role in the supply chain. Fast fashion has caused such a race to the bottom for price, everyone holds their cards incredibly close to their chest just trying to survive.
Sometimes shortening this chain and going directly to the person producing the raw materials is a much better option than getting it locally because it allows a better understanding of your supply chain. You can be sure that the product you are producing is ethical because you know who made it - start to finish.
Grower/Raw Yarn Producer > Fabric Mill > Your own factory > Your store > Customer
In some cases, manufacturing outside of your home country can actually mean you have more control over your supply chain. The goal of every brand should be fully understanding, vetting and controlling every level of their own supply chain.
Ask your favourite brands about their supply chain. If they manage all this themselves, they'll be very eager to tell you. If they're not too eager, it's likely because they have no idea where their stuff is really coming from.