I'm still gonna stick with school being one of the best places to get Controllers en masse. Or office buildings, but I'm less familiar with those, as I'm a teacher.
Students are, more or less, trained to listen to teachers. Obviously this doesn't always work perfectly, but it's how a student body of 3,000 is able to function with a staff of 300 (my school).
The hardest part would be infesting the principal of the school, but once that is taken care of the filter effect is fairly easily. Principal can call in his administrators (we have about 13 assistant principals) one by one, as well as his secretary, and forcibly infest them. He wouldn't even have to do it on campus if that would be too suspicious; administrators are always leaving for meetings. He could offer to drive one of them to a meeting or lunch or something and instead drag them to the nearest pool.
Once the 13 administrators are taken it would be useful to get the constables and general office staff infested - again, easy enough by a similar process. Then its on to the educational staff. Each administrator is over a certain department (English, math, special ed, gifted & talented, etc.) so they could call a meeting with their department after school. If necessary, at that point it'd be safe to bring Hork-Bajir in to help forcibly infest the rest of the faculty. One administrator plus two HB per room should suffice and not be too terrible of a force depletion.
Once all the teachers are taken it would be easy enough to stage a lock-down at the school, pretend it is a drill, and direct everyone a class at a time into the Yeerk Pool at Dracon point.
Bam. You have an entire school under control. Students can then go home and convince their parents to join this great new club called The Sharing and you have your in to a ton of businesses and industries crossing all salary ranges, professions and demographics.
