My mom taught public school for 35+ years and she ran into this problem all the time. Unfortunately technology (i.e. the internet) wasn't prevalent for all but the last 10 years of her career (and only the last 2-3 years in the county she taught in.)

As a teacher in the primary grades (in a rural county), she often had little or no budget for supplemental materials and would spend her own money (when she could afford to) in order to supplement the lessons.

Whoever was in charge at the time (at the principal and schoolboard levels) also had a direct impact on how much freedom she had to augment the textbooks. Generally speaking though, there wasn't much freedom at all.