I have to agree with the others LB, about someone local. Preferably from recommendation.
That's the difficult bit.
Timber sheds are generally made from tanalised timber... "treated" timber. If good quality, it would see us all out ;o)
The problem often lies, not in the timber, but the construction. They use timber with small sections (overall sizes and thicknesses.) Plus, they are often stapled together.......... rather too cheaply.
The only sheds I've had anything to do with have been ones I've built myself. If you want a properly pukka construction, then it would be worthwhile asking locally for quotations to build one for you.
Any good builder/handyman/confident DIY-er could do it.
Technical (i.e. "too much information" alert!)
A frame of treated 4x2 timber.
Clad with "feather-edged" board (as per good quality fencing.)
Proper Door/gate from a Builders' Merchants.
Cheap f/edged board is usually cut from 6x1. This finishes at around 3/4 of an inch one end, to almost nothing at the other.
It can be ok, but usually doesn't last.
F/edged cut from thicker timber is vastly better.
I'm only giving you all this technical guff so you can appear knowledgeable to whoever's quoting.
It'll cost more of course, because of the labour, but perhaps give it a thought.