Do they make sewer jet nozzles for garden hoses?

No, there isn't enough volume to make them possible. Your best bet of cleaning is to tie a rope to a Brick (if heavily clogged) or Cinder Block (if lightly built-up). You throw the scraping item in as far as you can (from both ends) and drag it back out to discard the debris.

- Repeat until mostly clean. Another option is to put the head of a Garden Hoe onto a swimming pool Cleaning Pole and use that to scrape the build-up out. You want a small amount of removal with each pass so your scraper doesn't get stuck and you can still pull the debris out.