Friendface - S3-E5
Other mistake: In the beginning of the episode, Roy is explaining why Friendface is bad and chastising Jen for being on it. If you look at his monitor, he is on a website with a green color palette like the Friendface infomercial and a layout similar to MySpace. While I can't be 100% sure, it seems like a graphic designer created a mock website of Friendface to be used in the show. If true, that means Roy is already on Friendface before Jen even enters the room. (00:01:00)
Answer: The shirt is available from www.ThinkGeek.com and is fairly popular in the nerd culture, so it's possible you've seen it in real life or online.