Possibly someone is still supplying it but under one or more of the synonymous names 'Columnare', 'Erectum', 'Sentry' or 'Monumentale'. Otherwise there are less strictly narrow but still slender newer cultivars being offered.

Here is a link that might be useful: JFS Introductions

Today I saw a 'Newton Sentry' at a local garden center, which tells me that it is in fact still available wholesale - probably in this case from an Oregon grower. Next I would check the online Oregon Association of Nurserymen Buyer's Guide to see if there are any listings there.

gee farms in stockbridge michigan carry these maples and ship all over usa incredible stock just saw them this am
this nursery is amazing 75 acres of the most unique full size go to gee farms .com you can download their whole catalog

If you can't find the maple you want, you could always try a Ginkgo biloba. There are two very columnar varieties. G.b. 'Princeton Sentry' and G.b. 'Tremonia'. Tremonia is the most columnar of the two.

See link: http://www.esveld.nl/htmldiaen/g/gibtre.htm

Regards,

K4