If you have an iPhone, getting a keyboard case is easy. If you have any other phone….not so much. There are plenty of cheap, phone-sized keyboards out there, but I've tried those and the rubber buttons are not good for typing on. There is however a tiny Bluetooth keyboard out there of higher quality, one that you could probably head out the door right now and locate rather quickly: the Sony PS3 Keypad.
That keypad is pretty cheap these days, and being an official accessory it's available in most game stores and other places that sell PS3 equipment. It's designed to connect to a PS3 and attach to the controller, but actually works out of the box with other equipment, both Android and iOS-based. You enter pairing mode by turning it on while holding the left shoulder button, and then just scan for Bluetooth devices on your device, then follow on-screen instructions.
The keyboard has a standard qwerty layout with round buttons separated from each other with plenty of space. There are two shoulder buttons which work like function buttons on laptops to access secondary and tertiary functions for the buttons. There are also a couple of PS3-only buttons that will do nothing for you, and finally a button that activates touchpad mode. This is a peculiar mode where the button surface acts as a touchpad that you slide your fingers over. On Android this brings up a mouse cursor, but the accuracy is a bit too random to be of much use.
As for the keyboard itself, well, this entire post was written on it. It's a small keyboard - a bit too small for my hands really, but the buttons themselves aren't bad. Keep in mind that the actual letter keys are pressed into an area small enough to correspond to portrait mode on a larger phone (Galaxy Note, maybe even S III and HTC One X), so compared to a keyboard made for phones in landscape mode it's really small.
The biggest issue is perhaps that it's not attached to the phone, so it's always something extra to drag with you. It's perhaps most useful in a setting where the device is docked, and in those cases you might as well have gone with the full sized Bluetooth keyboard. There's also the peculiar shape and attachment doohickeys for the PS3 controller to deal with, and even if you cut off as much plastic as possible (like I have) you end up with a peculiar thingamajig.
All that aside though, I think this is one of the best such keyboards out there. Between the price, availability and (build) quality the other issues are forgiven compared to those rubber keys keyboards that otherwise look better. A seller on Amazon has this keyboard for $24 right now, which is about what I paid here in Europe, but I would definitely think it could be had for less if you look around.