French Fuse is made to back T shirts. It's a tricot interfacing, so it has a little give to it, like the T shirts, but it firms them up well without making them stiff.
This is where I buy my tricot interfacing for T shirt quilts. It's the best price I have found:
https://www.ebay.com/itm/5-Yds-New-W...8AAOxyTjNScYZ6