WHERE TO GET THE GELS: the book you found was probably a Rosco sampler. the gels are used in theatres to changes the color of the lights on the stage. when i work on shows, i can usually just swipe them from the workshop (even though they're kinda expensive- hehe. oops a girl's gotta craft!!) so maybe if you go to a theatre and say you need a square-inch piece of Rosco gel in whatever color (it might be a good idea to go here and find the NUMBER of the color you want: that's generally how they are organized in a theatre.)
since the gels start off square, and lights are round, they'll usually have little trimmings lying around they'll just be throwing away anyway. huttah!
my latest Rosco project: inserting a little pattern in my cell phone case. just cut out a screen-sized piece and then chop it up and tape it in with scotch tape. very cool effect here, using a VERY light gel.