Good sources of filament "sampler" pack?
Amazon, of course.   I found a pack of 20 colors, maybe 50g each or so, 1.75 PLA. (that link is direct to this product).

edit:

Well, dang,  I blinked at that particular item is off the list.  Here are two other multicolor packages currently available (2PM EDT 20 Oct 2016)

one , and  ...    two