If you don't mind being a bit sneaky, try adding a couple of extra tills to your existing consignment store or a couple of custom consignment stores. Three tills means three times the chance of finding the item you want. Yes, each consignment clark will have a slightly different list.
If you don't mind cheating, you will find it in buydebug.
Actually, I've been playing this game for ages and I've never bothered with this invention. I always sell it after my sim has made it, but then I keep my gardening to a ninimum anyway. Just enough to keep my sim in life fruit and flame fruit. I use reading and harvesting wild plants to increase my skill to the point of being able to plant rare and special seeds and by then I nearly always have an example of these two fruits which I can plant alonside a couple of special seeds, no more than 2 or three at a time in order to get a couple o death flowers.