you can look at these:
http://www.unicyclist.com/forums/showthread.php?p=306888
http://www.unicycle.com/shopping/shopdisplayproducts.asp?id=2&subcat=24&cat=Rims
Although the Alex x rim is available in 16" it’s a road rim so that might not be too crash hot for trials.
http://www.unicycle.uk.com/shop/shopdisplayproduct.asp?catalogid=57
That tyre is 2.125 which is close
thats all i had time for, probably isnt a help but there it is.
what do you want a 16" trials unicycle for, 20" is much mroe standard and would be better i think.