Yep, I love 'em too. It's the main go-to orange in Italy (where you first saw them). I first had them in Asia (I think they originated in China a bazillion years ago).

There are a bunch of different varieties ... some sweeter than others ... but I think there are three main varieties. I believe they are also the healthiest citrus there is as far as vitamins, etc. goes. I think a few growers have them down here, but the U.S. market comes mainly from Texas and California. They come in season at the end of the year (Nov-Dec) and are at peak around now, then dwindle out around April I think. You might be seeing them locally at peak market when the prices are the most reasonable, but they should still be around for another 2-3 months (at most). You'll just have to search for them.