With the soft box, you can clip the gel inside the box. However, the gels we offer are designed more for 7-8" reflectors rather than larger softboxes or beauty dishes.
You can ususally find gels that are 20x24 or so at theater supply houses, and are less than $8 a sheet (typically). One of these and some gaff tape would easily cover a beauty dish. A couple taped together would cover the front of a soft box. For large items, the do sell gel by the roll.
A decent discussion about this can be found here:
http://strobist.blogspot.com/2013/01/ho ... urces.html