Here's what I've been doing for fake plants that sit in gravel, or plants that I do not want to put in containers.
I take a clear suction cup, they come in all sizes and you can buy them in the hardware and craft sections of the store. I take off the hook. Using a drill with a small drill bit (depending the size of the plant stem) I drill a hole in the top of the suction cup; not all the way through, just about half.
Next, I cut the stem of the plant off of the base it comes with (make sure you don't cut the stem too short). Then I shove the stem into the hole I've drilled in the top of the suction cup. It should fit firmly. Then I seal around the connection with aquarium sealant and let is set for 24 hours.
Now you can attach the plant to the bottom of the aquarium and it will stay very secure. It does not move when you vaccuum the gravel and the fish can't pull it up. The suction cup and sealant are clear and hardly noticeable. It works really well with bare bottom tanks too. Especially if your using glass stones because the plastic rock that the plants come anchored to clash with the glass stones.
I have looked for plants that are anchored this way in the store but have never seen them. So I don't know if they can be bought this way or not. But this really works great for me.