I just bought the red one yesterday at PetCo. for 11$. It's nice and small for the scary car ride to the vet's office. I just put in a little pillow case for her to hide in. I just wish there wasn't a zipper on it because when she hears me about to open it she runs to the door and I have to be careful not to get her fur caught in the zipper.
Shortapsgirl, if you are good with a sewing machine, you could probably make one. I'm pretty crafty myself, so I understand wanting to make one rather than buy one .
I would go to a store that sells them to check it out and see how it's put together. You probably need something fairly rigid to hold it up inside, I would suggest maybe plastic canvas. I use it for hat brims and it is stiff enough yet flexible. The outside you could do in whatever fabric you want, but you should probably do the inside with some sort of vinyl or waterproof material. Then you can put a little blanket or something you can take out and wash.
I made one out of spare grids and zipped tied them together into a small 1x2 rectangular box. On the top I attached grids so that it made a lid and use a clip to hold it all shut. just throw in some old towels or a blanket and you are all set to go. You can even attach things to the side of it for the trip.