Regulation has contributed to the trainee issue. It is not the single shop fee appraisers that have "decided" to not take on trainees and fix the supply... that is ridiculous.
It used to be that properly trained trainees could perform inspections. If you were a volume appraiser, it made sense to take on an apprentice, train them, and put them to work. You didn't have to waste a day driving around taking photos and measurements. When FNMA said that trainees could no longer do that, this extinguished the feasibility of having a trainee, which was now reduced to a report typing liability. They then re-allowed trainees to perform inspections, but most lenders don't want that. Can't keep changing the rules every few years.
So essentially the trainee is a useless liability to the fee appraiser. You should look into a larger appraisal shop or AMC, they are out there and they will put you to work... if you are competitive, that is.