I used tricot interfacing and bought it at Joanns with a 50% off coupon and bought a whole bolt! I forget how much it is but you can check their website. Just make sure that the "stretch" in the tricot goes the opposite of the t shirt stretch when you iron it on. I always cut my t shirt block bigger and after ironing on the interfacing I trim it to size. I use the tricot for the softness but there are other light weight interfacings out there that may be cheaper.