Here are a couple ways:
1. Most corp offices are opened on Saturdays. If these corp offices want you to work more days, you may have to leave your current job and work more days (including Saturdays) for them. To get 6 days/week work, they may make you travel to another brand office of theirs.
2. Buy a cheap slow dying office for under $150k. It's your practice; you can work on any day (Saturdays and Sundays) you want. When she first started her new office from scratch, my sister worked 3 days/week at her own office and 4 days/week for the corp office. Since most patients prefer weekend appointments (they don't want to miss works) and with fully booked weekend schedule, the weekend productions are usually 2-3 times more than weekday productions. So one dentist who works on weekends, 6-7 days/week can make nearly as much as the combined income of 2 dentists, who only work on weekdays.
3. Find practice owners with Vietnamese, Chinese, Indian last names. Look at the craiglist listings in your region. When I was a GPR resident, I worked 2 Saturdays/month for an Indian dentist. When my wife did her 3 year post grad residency, she worked every Saturday for 2 different Vietnamese dentists.