I took a trip over to my local Apple Retail Store and told an employee there that I had misplaced one of the silicone ear tips. At first, his canned answer was that Apple doesn't sell the silicone tips separately from the entire Apple In-Ear Headphone set. But, after pressing him a bit (asking if there were any spares in the back workroom, mentioning that my mom had received a replacement set from the Apple Store about a year ago, etc.), the employee relented. He walked me over to the shelf where there were neat rows of Apple In-Ear Headphone boxes. He opened one, handed me the small canister of silicone ear tips contained within, and mentioned that he'd DOA the newly opened package.
So, longish story much shorter: I was able to get free replacement silicone tips at my local Apple Retail Store. Color me pleased! 🙂