A few other dealers do the same as B&H - Adorama Camera (also in NYC) also stocks an extensive selection of pre-configured Macs. A friend just came up with an iMac Pro 64 GB/2TB off the shelf from them (B&H also had it in stock, Adorama offered a special - check the Apple Insider price guide since one or the other of them is often discounting even newish Macs). Both B&H and Adorama have good in-stock indicators on their websites - if they say something is in stock, they'll get it out to you the same or next day - this is a huge advantage over Apple BTOs that can take weeks. They also have extensive stocks of prior year Macs, with many configurations of the immediately preceding models, and a few a couple of years old, often at attractive discounts.
Both B&H and Adorama are extremely honest and reliable - I've used both for computers and camera gear for years. One thing to note is that both stores are owned by Orthodox Jews, and close every Saturday for Shabbat, and for extended holidays several times per year (you can't order online when they're closed, although you
can save a cart and check out later). Both of them give plenty of notice of upcoming holidays on their websites.
Small Dog Electronics (Vermont and online) at least used to stock a bunch of pre-configured Macs - they've changed their website so that it's now difficult to tell what's actually in stock, and what they'll just order when you order it.
Unfortunately, most cities no longer have a Mac dealer that stocks anything beyond the Apple configurations. Even in a big city like Boston, I can't think of one that is accessible to non-affiliiates of the universities (I suspect many of the university bookstores stock some extra configurations for students and faculty, especially at this time of year).
Unless you live close enough to New York to actually walk in to B&H or Adorama, your best bet is probably to order online from one of the two. The Apple Insider price guide is a useful resource to find out which of them is discounting what right now, and is also useful in that most configurations that show a price instead of "place order" turn out to be in stock (not sure this is 100% reliable, but it's pretty good). Here's the price guide link:
https://prices.appleinsider.com/current_gen