Ebuyer.com has six designs representing Star Wars, Doctor Who, Star Trek, and more. You can download the PDFs and print them instantly; here's a few examples:
|(you can draw on the extra "E" in Wookiee")|
Rachelle Abellar has created a whole collection of awesome Doctor Who Valentines. You can download and print them for free, but if you like her work, you also have the option of donating!
From Rhonda at Dollar Store Crafts:
Karina has shared a whole bunch of Avengers, Sherlock, and Supernatural Valentines at Tumblr. They're not downloadable, but you could print some for personal use!