1-3 Mibo makes retro-inflected tile decals in a variety of colors and patterns. $18-20 for a set of 6 at 2jane.
4 Berlin Wallpaper makes their decals subtly three-dimensional, so they feel like real tile to the touch. They have several designs, but my favorite by far is the Delft pattern. $10.99 for a set of 4.
5 Sticviews has a gallery of patterns and styles, including this geometrical "wrought iron" pattern, but you can also upload your own images or non-copyrighted images from the internet and they will custom print them. I would find a great Iznik pattern online. Price depends on size and quantity of tiles.
6 Karen over at karensvinylvillage.com will also make a custom tile design for you, per a recent blog post.
7 Spin Collective in the UK makes tiles stickers in simple motifs, like fleur-de-lis, birds and stars, in a wide range of colors. £20 for 36 stickers.
Images: as linked above