[Fordors]

You could look into Maker Workshops, there may be one in your area and some have a pretty decent machine shop. They might have a vibratory finisher. There are different shapes of ceramic media, cylindrical cut on a bias, triangular, and some applications even use metal media. For the tight corners on a manifold I’d probably want cylindrical, bias cut and maybe about 5/16” or so diameter.
Depending on how aggressive a machine is they can be pretty noisy, the one I’m familiar with was used to de-burr sintered metal parts, was about 4 feet in diameter and had a heavy acoustic blanket on it to dampen the sound.