Most office supply stores (Staples, Office Max, etc) sell the DIY T-Shirt transfer "kits".
Design your own graphic, print to the special iron on transfer "paper", than iron onto the fabric of your choice. I believe Canon is one manufacturer who makes the transfer "paper".
I did this several years ago. Still have the T-shirt I "made".
Another option is to use the vistaprint.com promotion. A "free" custom printed T-shirt. Upload the "logo", pay S & H. I believer it's $4.99.
I can just about guarantee they have one at the Bonanza Gift Shop "The worlds largest gift shop" on the corner of The Strip and Sahara. You can call them at (702) 385-7359 (Ext #2) to see what they've got...I imagine they have a wide variety of T-shirts and Coffee Cups with Area 51 themes.
I remember seeing them shirts at one of those cheap T-shirt stores. You know, the ones with the bride/groom game over, support single mothers (dancing on a pole), choking hazard and other stellar examples of low-life humor.
The Little A'Le'Inn in Rachel, NV offers a variety of t-shirts and other Area 51 memorabilia. The Inn is located on the Extraterrestrial Highway at N37° 38.801?, W115° 44.760? on one's GPS.
One can also purchase the Area 51 T-Shirts from them online.
"Watch the skies, everywhere! Keep looking. Keep watching the skies!"__Ned "Scotty" Scott in The Thing From Another World (1951)
I remember when I was abducted the first time, I ask these suckers for a tee-shirt, and they said they weren't in the "commercial" business of selling them. They told me to try Wright-Patterson AF Base. Never got there though and they wouldn't give me a ride!