1. Currently under Custom Mock Drafts you will see a 12-teamer with 4 spots left. Open to anyone. When it fills it will start, so you have to watch for that. That has an up-to eight hours to pick for one turn, so may not be ideal.
2. Once or twice a week a Razzballer will initiate one and post the details on Razzball. Tuesday nights have been a frequent time. You can see these under Custom Mock Drafts, 2 minutes for a selection, and may/may not require password.
3. The Fast Live Mock Drafts you can get in by the hour. Seldom will you find more than a few players. However, it is an excellent way to get familiar with the nuances of how the site works and the feel/problems associated with 90 second allowance for drafts. With the computers filling in picks rapidly, it can be harrowing the first couple of times.
Couchmanagers has the advantage of while you are drafting you can edit in which position you want players. Disadvantage: they don't provide after draft projections for how your team rates in roto.
Mock Draft Central just dumps your player to their "primary position." Sometimes to the bench even though you want him starting. And, you can't change it. Also, even though they provide projections, they suck. I would never use that tool to evaluate my picks nor decide to shift selections based on it.
Get in the water!