I love passion fruit and even here it can be hard to find. A lot is grown on the Big Island but a high percentage of it goes to Kona Brewing for their Wailua Wheat (lilikoi) beer. My guess would be to find a South American or Caribbean grocery store. I have no idea about the season. I tried looking up a source online but it proved to be more difficult than I thought it would be.
I love the Häagen-Dazs passion fruit ice cream too. It has such a pure flavor and is so creamy. It is amazing what just five flavors can do for an ice cream. Nothing artificial makes a big difference. Have you ever tried a passion fruit before? We were lucky enough to have a vine in the back yard of the place we stayed at last Christmas. Although the fruit was not quite ripe the flowers were stunning. Here is one of the photos I took of them.