Hey Melik,
The only Muppet building plans are the ones you experiment with. I can only think of one pattern that's ever appeared in any Muppet book, which is of Rowlf the Dog, in "Jim Henson the Works" on page 32. And thats just a sketch of a patttern. I really doubt you'll ever find an actual blueprint of a Muppet puppet outside of the Henson company (copyright and licensing issues and such!)
Check out my webpage in my signature below to see what I did just by experimentation.
What are your plans for puppetry?