Libby,
Try your local surf/SUP shop. They may be able to work with you on a trade in for a new board if you don't want to deal with craigslist or forums. Plus they'll be able to tell you about demo days, and places to offer rentals, so that you can try a few different shapes and sizes before you make a decision.
The brand is really more of a personal preference. The real thing you need to look for is board volume. The wider and deeper the board the more side to side stability it'll have. 10'6" should be a pretty good length, and I'd recommend a width of 30"+ for what you're looking to do. Also that flat bottom surf board style board will help keep you nice and stable for yoga. Plus they'll turn in tighter areas easier.
Remember though the more time you spend on any board the more comfortable you'll get and the more stable you'll be. I use a similar board to your Ohana and can hold most yoga poses pretty well on it (my flexibility limits me more than the board). I'm also on the water at least 5 days a week though. The nice thing about a "race" board is that it glides really well. So if you want to do some long distance paddling it'll be a lot easier on you.
