I know Aircraft Spruce has them, but you can probably get them for less if you search around for surplus. Look for D-436-(xx) as a part number. The (xx) is the size: 36, 37, or 38. Red, D-436-36 is for 20-26 ga. Blue, D-436-37 is for 20-16 ga. Yellow, D-436-38 is for 16-12 ga.
I bought my crimpers on E-bay. Look for "Raychem AD-1337". Patience is your friend here. I've seen them go for as much as $200 or as little as $50. In pinch you could also use the Chanel-Lock crimper/wire cutter from Lowes. They work on the blue and yellow, but you will not get the same quality crimp. The truth is if you are going to spend the money on these you might as well get the right crimpers.