If your stores don't carry it, here are a couple of options. Ask if the store will order it for you.
Second, Amazon.com has it online, order and ships to your house. Good luck!
But if you want to make lemon gelatin, and there's none in store, use unflavored & add lemon juice, zest, extract.
You may not be finding lemon gelatin in stores because it is one of the things you CAN eat the day before a colonoscopy. Nothing solid, milky, red, orange, purple or green. The store may have been bought out of the lemon flavor.