Crockpot on the lowest setting, lid cracked, should be right around 150 degrees. 4 hours with the lids tightly on - done! (And usually even better the next day)! You can get a small crockpot from Target for $10
Utrasonics tend to have shut off times, which makes leaving them in there for any length of time annoying, because you have to keep resetting them.
That said, I rarely steep any of my DIY, but occasionally the crockpot is a lifesaver!