The first two movies in the franchise are, undoubtedly, the best, and the original Terminator film from 1984 is often regarded as the best sci-fi film of the 1980s. A fast-paced storyline, jaw-dropping action sequences and, of course, Arnold Schwarzenegger's perfectly cast role as the cyborg sent back in time to terminate Sarah Connor, all combined to create an iconic 80s movie. 
One of the best-remembered and well-known quotes from the films is "I'll Be Back!" (which was first used in the original movie after the terminator is refused access to the police station) and you'll find a plenty of T-shirts with this theme. 

Simplyeighties.com is a participant in a number of affiliate programs which allow websites to earn fees by linking to their products.

Best places to buy Terminator T-shirts
After spending some time searching the web, I discovered that the following suppliers had the most comprehensive selection of Terminator related shirts;
ebay (UK, US, Canada, Australia)
Redbubble (Worldwide)
Tostadora (UK, US, France, Italy, Spain)

Here are the tees I found at Amazon UK. The datafeed below is live and many of these products (and more) can also be bought at the U.S. store.