Much like most things on Roblox, you will be making this yourself from scratch, so you'll need to design a t-shirt in an image and then upload it to Roblox.

How to Make T-Shirts in ROBLOX T-Shirts are the easiest clothing to make in ROBLOX. Create an image that’s ideally 512x512 pixels and then upload it to ROBLOX via the Asset Manager in ROBLOX Studio.