Quills come in all sorts of stem length and height too, so if you really want a quill stem, there are plenty available. Your local bike shop or online retailer have them at reasonable prices. You have to know both the diameter of the part going in the headset and the diameter of the handlebars where the stem grabs them. 25.4 is only one of those.

However as a previous post mentioned, it is worth considering getting a quill adapter for threadless stems. Much easier to change your stem lengths or angle of stem if you later decide the current one is no longer what you need. No need to remove levers and bar tape. Little bit more cost initially, but less trouble long term.

edit... oh as I often do, I didn't quite understand your post. I guess you are asking about change to a pista type stem. So if your bike current uses a quill, then look for a "quill adapter" and whater threadless stem suits your fancy. You'll have to know three diameters though. the diameter going into the frames headset, the diameter of the mount for the stem and the diameter of the mounting area of the handle bars. As well you'll need to be aware of stem length and stem angle. You can put most on either way to give you a negative angle and a positive angle.