You would probably just need to find a screen printer (by calling around locally or posting an ad in our referrals area) that can source blanks with tearaway tags wholesale and screen print your design in the neckline.
It's not uncommon at all. They will probably want to screen print your actual design (on the front of the t-shirts) as well.
If you just want someone to supply the blanks and print your logo/brand in the neckline, that might be hard to find.