You can make your own non-skid using Imron urethane paint. Carefully mask the area where you want traction. Masking tape must bind very well to substrate otherwise Imron will wick under tape. Using foam West System rollers, roll on one coat of Imron. Immediatel roll on a second coat of Imron, then immediately roll on a 3rd coat of Imron being careful to get a very smooth, even application of the paint, and right behind the roller on the 3rd coat sprinkle sugar very smoothly and evenly. You can strip off the masking tape about 1 hour after the sugar is applied. Let textured area stand about 16 hours then rinse off sugar with a water hose using fresh water. It will leave a durable non-skid textured surface.
You might want to practice in a non-public area before trying it out on the deck where everyone can see the results. I can't ephasize enough, the masking tape must be tight to the substrate, the third coat of paint must be very even, and the sugar must come right behind the roller on the 3rd coat. The sugar must be applied very evenly. It's really easy to do, but you must be focused.
Be careful to apply the sugar in a nice even smooth application. I like to use a salt or spice shaker. The pattern you create with the sugar is what you will see after the sugar is washed off. Let dry and harden for about 3 days before giving it heavy use. If the texture is too harsh, you can sand lightly to ease it. Imron is one of the few paints which is tough enough and doesn't tend to "yellow" over time.