The ply is for the day job rather than the hobby. I work for a company that makes military shelters, one of my team is looking for ply to make work tops for deployable machine shops for the British Army. and the designers have come up with the idea of using 1 1/2 work tops its now down to purchasing to sort it out.
2 X 18mm laminated together would be my solution to a one off requirement, although presumably thats not seen as a realistic option for mass production?
If it comes to it we will press shhets together in our pannel press, but that is added labour cost so would berfer to buy off the shelf. I have come up with 35mm and suggested an alloy shin for the top to prevent damage.
I seem to remember some years ago when I was at Robbins timber in Bristol http://www.robbins.co.uk/ they had what looked like extra thick plywood - almost 50mm. They have a pricelist for all their timber on the website http://www.robbins.co.uk/pdf/Retail_Pricelist.pdf but I can't find 38mm only 25mm listed.
You could always give them a ring and see.
I found them a useful source for decent plywood (down to 1.5mm thickness) as well as faced ply. The catalogue is worth a quick look and is one of the few places that actually publish prices for all their stuff. They also say they deliver anywhere!
If you're planning to go down the route of laminating sheets together, wouldn't you be better off with three sheets at 12mm?
Two at 18mm seems unbalanced to me, you may find that the sheets would want to cup, bow or distort once the glue cures... Even if you look at the total number of core and veneer layers (seven each?), you'll still be looking at an even number over all. :? :?:
For a start, ply always has an odd number of layers os that both exposed faces have grain running the same way. Laminating two together will either give faces with grain running in opposite directions or two consecutive layers going the same way in the middle. Three layes will maintain the odd number of layers and so give both exposed faces going in the same direction.