Brian Goulet’s Swab Shop is an excellent resource. However, Please also read Margana’s post at An Inkophile’s Blog: Beware Online Ink Swatches! It’s important to know what you are getting into before you get started. Seeing an ink on the computer screen is not that same as seeing it in person.
Two ink retailers offer ink samples: Brian at The Goulet Pen Company and Ryan & James at Pear Tree Pens. (Note: Pear Tree Pens is now out of business)
There is also the FPN Ink Sample Exchange.
Finally, for another source for online pictures you can browse the FPN Ink Review Index. Also, you can Google whichever ink you are interested and find various blog reviews.
The Dizzy Pen 
Todd Nusbaum at isellpens.com also has ink samples. I believe that Pear Tree is out of business?
This is my first time of buying ink samples and I love it. I’ve always just bought based on the online reviews, but it’s wonderful to really test out a color before buying.