A 2-inch diameter checkvalve is not something you're likely to find at your local home center, but I found one online: a “true union swing check, clear, 2.0"” cost $31.99 from Koi Joy. As the pump is near the level of the water surface, rather than near the bottom of the pond, as some people prefer, the checkvalve, which is simply an enclosed flap that allows water to flow only toward the pump, is required to maintain the prime when I turn off the pump.