Hi Mark,
Welcome to SailNet!
I have been using the life vest provided buy the rental locations. But I would like to buy my own. What brand / type would you recomend?
West Marine has a nice article to help one select a life vest. Here it is and their section about choosing a life vest for day sailing:
HOW TO SELECT A LIFE VEST
West Marine is committed to outfitting your life on the water. With over 250 store locations, 100,000 products in stock, and knowledgeable Associates, trust West Marine for your boating, sailing, fishing, or paddling needs. Shop with confidence - get free shipping to home or stores + price match...
www.westmarine.com
How to Choose the Right Life Jacket
Day Sailing
Sailing small boats requires PFDs that fit snug to the body and that maximize freedom of movement. The preferred style has a zippered closure and is made from soft, pliable foam. Day sailing vests often feature an articulating design, with strips of foam inserted into channels that enables the vest to flex and wrap comfortably around your torso. Large armholes offer additional freedom of movement, but may allow the vest to ride up when in the water, so a snug fit is important. If you sail a dinghy or beach catamaran, your PFD may have to be worn with a trapeze harness, so take the harness with you when you shop for a new life jacket. Day Sailing vests are a good choice for a variety of boating styles except for high-speed water sports.
Naturally

they have a selection of PFD's they categorize as Day Sailing Life Jackets:
Day Sailing Life Jackets
Shop sailing life jackets at West Marine including low profile life jackets and more from Spinlock and others. Get free shipping to home or in-store!
www.westmarine.com
That might help you find what you are looking for.
~~ Red