I have some fantastic elm boule, all superb quality and a lovely red colour; kiln dried, sizes from 27mm to 80mm, about 40 m3 available. No Dutch elm disease here! Elm wood was valued for its interlocking grain, and consequent resistance to splitting, with significant uses in wheels, chair seats and coffins. The density of the wood varies due to differences between species, but averages around 560kg per cubic metre. The wood is also resistant to decay when permanently wet, and hollowed trunks were widely used as water pipes during the medieval period in Europe. However this resistance to decay in water does not extend to ground contact. (I did not write the above, my web designer found that, as I had not got much to say about the Elm, apart that it's pukka!)

Green oak logs are readily available and can be cut to your specific sizes. Green oak has shown a large increase in popularity in recent years, due to its enormous constructional strength, great resistance to decay, wonderful character and colour. It is also competitive against steel and widely available from soundly managed sources on the continent. 

Less abundant in France than the UK, I have done a few trials, usually in 27mm and 54mm, if you are after something specific please drop me a line. I can source any thickness and either in kiln dried or air dried, boule, square edge, but I do need some time on this, as it's incredibly peculiar over here in France. You ask some mills for pippy oak (we call it patte de chat which translates as cat's paw) and you end up with complete rubbish with big holes in the boards, and sometimes it's clean as a whistle, so I am on the permanent hunt for the right log. I usually get the pippy oaks milled at 27mm and 54mm, and have also begged for some 80mm recently, that way it can de deepened.