This is the Doc Johnson Mood Naughty Silicone Butt Plug. You can get them here, or any number of other places. I actually got mine from another online behemoth that surprisingly carries much more than just books - it was a little cheaper and had free shipping! They come in black or pink, and are available in small, medium or large. On the size issue - if you have ANY kind of experience with butt plugs whatsoever, you will want the large. Seriously. Don't be scared by that. The large is smaller than another medium I have. From what I have read, the small is extremely small. Pinky finger size. So if you are new to the idea, that might be the one for you. But I have real concerns about it staying put, and there is nothing less sexy than a butt plug that pops out! The large gives a nice full feeling without being painful.
This is a silicone plug, and the feel of it is soft and velvety. Silicone means it is easy to clean - you can even put it in the dishwasher! Just make sure loading and unloading the machine is your job. (You can also put a condom over it before insertion if you like. Clean-up is literally a snap.) It also means you must use a water-based lubricant with it - silicone lubes will break it down. I like Astroglide.
The shape of this toy is great. The slim neck makes for a secure fit and less chance of it popping out than one with a more tapered neck. And the base is wonderful! Not a round base that will poke and push at your cheeks when it is in place, but one that is shaped like a handle that nestles nicely between the cheeks, making it more comfortable for extended wear (like those shopping trips). The handle is also convenient for your partner if they are in a playful mood!
All in all, this is by far my favorite toy. If you like this sort of thing, I highly recommend it. 10 out of 10 stars!