I buy the Rosco color correction filter pack
. The sheets are 10x12 inches, and come in various strengths of CTO, CTB, magenta, and green. Costs about $30. I just cut the filters to the right size and tape them over the front of my speedlight head with gaffer's tape. I get a lot of filters out of this pack. You can make precise filters by mixing, say, a Half CTO with a full plusgreen filter for some types of warm fluorescents.
There is always a Full CTO filter taped to the back of my flash for immediate use when needed.