That is a great little mech, but I had the same situation. After years of use, certain parts just wear and of course , no replacement parts.
You might have to see if anyone chimes in with suggestions for compatible parts you can use. There are not too many side switches around anymore.
Guess the mechanism was more prone to wear and maintenance than a bottom button/ switch.
You might keep an eye out in the classifieds for an Atmizoo mod. They had the Dingo , The Lab, and the Guppy. Side button but at the top end.
I never had one, but always wanted one. Only heard great things. All the ones I mention were non-telescopic but were modular, so you could have it any size you wanted.
The other one that was wonderful and had a side switch were the Polidiacs, again amazing device but no longer made.
The GG ( GoldenGreek ) mods were the Rolls Royce of side switch mechs, but were telescopic for the most part, but they did have a few models that werent.
Every once in a while I do see The Smok Natural on Ebay but the price is either ridiculously high ( over 100 ) or down in the 20's
FOUND one for you at Madvapes
http://www.madvapes.com/the-natural.html