It may sound a bit weird, but you can get good powder for use in cobs and other devices from bottle rockets. Not all bottle rockets work however. I did a test where I filled one cob with powder from Mad Dog whistling bottle rockets and another cob with powder from Air Traveler bottle rockets. The Mad Dog worked perfectly, but the Air Traveler didn't even explode, it just shot sparks out the hole. I had assumed that the powder in Air Travelers would be black powder, but apparently it is some other, slower burning powder. The powder in the Mad Dog on the other hand, is a white powder called whistle mix. Aside from whistling, it is also used as a break powder for shells (In other words, it explodes like black powder!). I have found this method to be much easier and cheaper than Estes powder, and it will be my main powder source for a while now. Keep in mind it takes a lot of bottle rockets, so buy a lot next time! Most (if not all) whistling bottle rockets have plastic casings, so that is what I will be writing for. If you have cardboard casings on your rockets, the procedure will have to be changed a bit.

Materials:

Lots of whistling bottle rockets. Any brand should work, but I especially recommend tri-whistle rockets because they have the most powder. There may be some non-whistling bottle rockets that have good powder; I have only tried Air Travelers. Do some experimenting!
Needle nose pliers
Two containers and garbage can

Procedure:

1) Remove stick and label and throw them in the garbage.
2) Remove fuse and set aside (you will have a lot of fuses after this!)
3) Squeze rocket body over a container with your pliers and powder will start coming out.
4) When orange powder starts to come out (This is clay! It won't burn!) hold it over your other container and get that out.
5) When silver powder (The flash powder, the report stage) starts to come out, hold it back over the other container and empty it.

Several things to remember:

There is a paper fuse in there also. The clay that separates boost and report phases gives the fuse time to burn for a second. This fuse is pretty much useless, but it's a waste of time to try to pick out each one as soon as it falls out, just get them all at the end.
Don't worry too much about clay; just try to keep out the majority of it.
Save the fuses, they are very useful!

Taking powder from bottle rockets:

Piccolo Pete's go by several names, including Whistling Pete and Screecharoo. They contain pure whistle mix. Different kinds and sizes have different ammounts of powder. I got a 12 pack of Piccolo Pete's to remove the powder. When I took them apart I was quite dissapointed by the ammount of powder inside.

Piccolo Pete:

Inside the Piccolo Pete:

The powder from all 12 (not much!):

TNT Piccolo Pete (same name, different label, brand):

Inside TNT Piccolo Pete (more powder than other brand):