The Strobist website points us to three fine sources for small gels that photographers can use with small speedlights.
Photogels.com sells pre-cut color correction gels for small flashes. Each pack comes with 10 different gels so you’ll want 1 pack for each flash you are using. The gels come in 2 x 3 and 2 x 5-inch sizes.
At Amazon.com, the Roscolux Swatch Book sampler could be had for $2.50.