There hasn't been a lot of Sylphiel stuff made, and that's a fact. Oh sure, you can get Lina clocks and Lina lighters but you can never find a Sylphiel pin or keychain. For your personal reference, I've put together this non-exhaustive list of fanmade items you can buy that are related to Sylphiel. It will be updated gradually.
There are no officially made plushies of Sylphiel as of yet so if you want a UFO of the shrine maiden you're going to have to either sew one yourself or find people who make them. However, Sailor Moon Specialty Shoppe makes custom plushies for every anime series you can possibly imagine and to my surprise they actually have a Sylphiel one, as well as the rest of the Slayers characters. She is extremely adorable and once I save some money, I will order one for myself.
If you're one of those people who can't sew, not only does the Sailor Moon Specialty Shoppe have plushies, they also sew cosplay outfits at your request for reasonable prices. I paid about the same amount of money for the materials of my Sylphiel outfit that they ask for theirs. You'll have to pay extra though for plus sizes as well as the boots. Be sure to check out the rest of their site for even more Sylphiel goodies you can probably request.
While this figure is not made for distribution, I thought to include it since it was made for me as a birthday gift by my good friend Kit, who created this adorable figurine of Sylphiel out of putty. Feel free to e-mail her to see if she'll make one for you as well.