Answer: I would suggest you work with your local nurseries to see what they have in stock on hand or could obtain for you. There is a dealer locator at the Monrovia web site to help with contact information. If they are not available locally they should be able to suggest substitutes.
In a brief search, I discoverd Acer nigrum is not that easy to find. Forestfarm does offer two versions, Acer nigrum "Greencolumn" and Acer saccharum nigrum. (The black maple in some references is sometimes included as a subspecies of A. saccharum or sugar maple.)
Populus tremula 'Erecta' is included in the Monrovia plant listing and so should be easier to locate however I would caution you that there are some serious problems with poplars in general, beginning with their being weak wooded and prone to breakage in winter storms. If you need columnar tree your local nursery staff may have some other suggestions for you.
I hope this helps.
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