While some specialty stores stock Fiori di Sicilia, you’ll have much better chances and more consistent availability buying it online. King Arthur Flour sells a fine bottle that I find to be very close to the variety I’ve had in Italy. You can also find bulk volumes for food service.
As with vanilla and other extracts with wide variance in their flavors, the important thing is that you find a source with consistent flavor you develop familiarity with. Different brands can have different concentrations that affect the outcome of your product.
Is it worth it?
Fiori di Sicilia is both rare and a bit on the expensive side, at least more so than plain vanilla extract. Is it worth a place in your pantry? While I was hesitant at first to buy such an oddly specific ingredient, I actually would recommend it to any intermediate or advanced bakers. It turns out to be a really versatile extract that will get used—unlike that bottle of coconut extract that’s been sitting in your pantry for two years...
I’ve had people remark “there’s something i love about these cookies but I just can’t put my finger on it,” which is a powerful tool to have in your arsenal. It’s a subtle secret weapon that won’t steal the show from your recipes.
Can you substitute Fiori di Sicilia?
You can create a rough substitute by combining orange zest, vanilla, and a touch of lavender to hit the herbal notes, then extracting the essential oils with a liquor of your choice. At that point, it might just be easier and more cost effective to buy a bottle. Unless you are an avid DIY-er, I would recommend you leave it to the experts.