A few questions:
What kind of smoking are you looking to do? Mainly briskets/ribs?
Do you want to slow smoke sausages and other items in a vertical stack?
How much do you want to be able to cook at once? 2 briskets? 4 briskets? 4 racks of ribs? How many chickens?
Do you want it to be mobile or stationary? 3/8" steel will make a HEAVY smoker. I have a 20"x40" main chamber smoker of 1/4" steel and the whole thing weights in around 450lbs, and I got the builder to put some larger 10" semi-pneumatic wheels on it, and it is still a bear to move around in the yard to set up.
3/8" may be nice, but I am perfectly happy with 1/4" and it is more than heavy duty enough for what I want.
How much are you looking to spend?
Where are you located? Some places further away may be cheaper, but delivery of a 500lb monster will make them non-competitive. I got mine from Gator pits because they are 5 miles down the road, and it was nice to talk to the owner to get what I wanted and be able to see several of their units and the build quality prior to purchase.
I spent about 1500$ for mine, here is a breakdown of what I got for the money to give you some basis of comparrison:
-20x40 main chamber, 20x20 fire box,
-baffled with spaced 'tuning' plates under the smoking chamber to balance heat from the fire box
-they mounted my purchased custom wheels
The link below is to the smoker on their website:
http://www.gatorpit.net/backyard_pits/party_gator.htm
The only difference with mine is the wheels and the shelf on the front and over the firebox is one solid shelf, not split over the fire box.
I would be happy to send you more information of all the places I looked and the pictures of mine if you want me to, just post your email.