Hey, Jonathan!
I have one I built from 2 blocks of 4" foam (which I got from discarded couches on side of the road). You may have seen it, but if not, video is available at
www.suspiciousmindtrick.net about 2/3 down the site--it's my buddy Tim Brown and I performing "Feed Me," and yes that's our voices.
Basically, I carved a top half and bottom half of the pod, then pinched out the area to be the mouth, carved a tongue from 1" foam, tore the teeth out of WHITE 1" foam, and attached the back of the mouth of the plant pod together with contact cement and a pre-built puppet mouth hinge from corrogated plastic. I painted it with watered-down acrylic paint to get the air-brushed effect, and then attached all kinds of fake foliage and a puppet sleeve.
Honestly, it took me too long to build one to want to make another like it, and the man hours that go into it would make the cost likely above and beyond what you would find reasonable.
I would suggest that you attempt it yourself, and soon enough you will perfect it. There may come a day that I might decide to part with it (I plan on parting with my Slimer prop/puppet when I build a new and improved costume/puppet). But not anytime soon.
Good luck.