On a speedlight, the swatches that can be found for free or cheaply are big enough to cover the flash head. As you are in the US, Rosco gels are probably easier to find than Lee.
One source is B&H. https://www.bhphotov...Swatchbook.html
Another way is to use Rosco's dealer locator and find the closest dealer. Where To Buy | Rosco. Rosco don't handle swatchbook requests themselves anymore, so you have to go through a dealer.
With the naked flash, which WB setting works best will depend on whether the artificial light is the dominant light source or not. If you have the time, shooting a colorchecker at the most important part of the scene and profiling the light from that can give you the most pleasing result.