I picked up these bowls at the local grocery store. What made them nice was they had several different sizes and the price was right. I ended up purchasing a couple sets of bigger bowls and a couple sets of smaller bowls. The bigger bowls became the outside shape and the smaller set became the inner mold.
The hardest part was cleanly cutting the bottom off of the larger bowls. There are probably other was to do this but the table saw worked best for me. I raised the blade high and ran the opening of the bowl against the fence and slowly eased the bowl into the blade. I then rotated the bowl until the entire bottom was cut clean off. I then fixed the smaller bowl to a small piece of 3/4" scrap melamine. This was done simply by drilling a hole in the bottom of the bowl and then attaching it to the melamine with a 3" screw.
Lastly, I centered up the larger bowl around the smaller inner bowl and glued it down with silicone. Once the silicone set, it was ready for the concrete. The concrete always comes out of the molds easier with a release agent, I find that a decent car wax works really well. I gave the entire interior of the mold a good waxing before the pour. For the pots shown I used my own concrete mix but something like Quikrete 5000 would be plenty good enough for a project like this. Just remember, to much water will make the concrete weaker and more likely to crack. Use as little water as possible but just enough for it to be able to settle down into the mold...this can take a little practice. Another tip is you don't want the concrete to dry out fast, concrete does not dry, it cures and hydration during curing is extremely important for increased strength and minimizing cracks. I typically keep the concrete damp for a few days after the pour.
After I finished up the pots, I sealed the interior with a poly acrylic. I wasn't sure if the watering of the plant would make it's way through the pot. If you were going to keep the pots outside or on a surface that can't be damaged then I'm sure you could get away without sealing them. Angie really likes succulents so we picked a couple different kinds and they really looked great.