The actual maker of many of the tires sold by Universal, Coker and other vintage tire sellers is Specialty Tires of America
http://www.stausaonline.com/ They were the maker of the 1050-16 directional tires that we have discussed so much in the thread
http://www.mapleleafup.org/forums/sh...&threadid=2421 for example. There is no listing on their site for a 900-18 tire (interested to note that I didn't see the 1050-16 listed any more, perhaps they've sold all of the first run?).
If you can get friendly with someone at a large commercial tire seller, preferably an "old-timer" who can use a paper catalog and remembers what (and maybe more importantly where) he used to be able to buy unusual tires, you may be able to track down some original tires in the corner of a warehouse. Toronto is big enough, still with industry (not just administration), that this could work for you.
Also, you may want to try other forums aimed at commercial trucks and busses (and fire trucks). I suggest busses because I think that some busses in the 30s-40s used larger section 18 inch tires on the front to have capacity closer to the dual 20 inch tires at the rear while maintaining a similar diameter.
I remember photos of Ford 6x6 and 6x4 conversions for airfield use that were fitted with directional tread tires, size unknown. My memory is of a tire profile closer to commercial proportions than military. Next challenge is to find photos or references.