No answer but just a thought. With that qty you must have some idea of the brand of shirt you want. Talk to the sales persons for the shirt companies you are considering and tell them what you are looking for. Most sales people have a vast list of companies that can handle your needs.
As far as cost I really have no idea but I do know that the faster you want something the more you pay. If you want it overseas and cheap you could wait 4 to 6 months easy and 12 to 18 months if your product comes in wrong. Done that and now 30 months later we are just throwing it all away and buying US.
For the Shirts: Walgreens sells T-Shirts for between $2.00 to $5.00 each. I've seen them on sale for 5 for $10.00 and they have all colors. They are usually Hanes Brand, which is a good brand.
For the Content: Staples, Walmart, etc. sell "Iron On T-Shirt Transfer Paper". You print them through an ink jet printer like normal paper then you cut it out and iron it on to the T-Shirt. Spend the extra money for the "For Black/Colored Fabric" because that comes out clearest on the shirt.
You just cut them out, lay the t-shirt flat, put the transfer on the shirt, and Iron it according to the instruction on the package.
I've been making t-shirts for friends for years. Quality and longevity of the transfer depends on a LOT of factors...
The first set I made for my friend lasted him over a year of frequent wearing... Another set I made for my father with Squidbillies stuff on it lasted 2 washes and now all the ink is rubbing off.
I don't know why this is... So you might have to try using different brands of transfer paper in the long run until you get a feel for it.