That sucks for a location but on the other hand there should be plenty of businesses in that area. I'm in the Rochester NY area, thus the mention above of the local steel supplier, one of several in fact.
Google is your friend. A bunch showed up just searching for steel suppliers Westchester county. None of them advertised selling drops and surplus but it is hard to imagine a full service steel supplier not having drops. The key here is full service.
Yes getting out of the cellar is important.. Trying to sell such stuff on the web increases costs significantly. Getting a good deal on non prime stock means taking advantage of opportunities as they crop up.
That could be an issue; there is a lot of corruption in the NYC area, living expenses are high and I would imagine dead is high. If a vendor is charging his prices for non prime stock then there is something wrong.
I've never seen any of their hardware so frankly can't offer an opinion, but the web site screams stay away to me. How all of your components end up bolted together, welded together or whatever is very important in a machine build.
Well I'd have a hard time trashing something I paid for, but I understand the frustration.
There are a number of vendors that supply to the tool & die and machine tool building industries but I have to warn you sometimes the prices seem high. Some examples off the top of my head:
Misumi,
Travers,
MSC,
Champion Die-Tool,
PunchTools,
anchor-danly,
cnc-specialty-store,
Southern Tool,
Grainger,
KBC Tools,
Production Tool Supply,
Harbor Freight (sometimes referred to as horrible freight),
Wholesale Tool,
Share Tool Company,
Little Machine Shop,
Victor Machinery,
80/20,
Carr Lane,
https://www.boschrexroth.com/en/us/p...-framing/index,
Faztek,
T-Slots,
alufabinc,
Nifty Bar,
Brunner Enterprises
There are all sorts of options. For example you can buy 1-2-3 blocks ( and larger variants) pretty cheap the days as imports, mostly available from machinist supply houses. You can buy cast-iron angle plates also pretty cheap. The aluminum extrusion suppliers have all sorts of predrilled or punched connector plates, gusset plates angle and such. If you can pick up some angle iron drops and a good hack saw you can have fairly decent but not completely square 90 degree brackets.
Most of the shelf bracket I've see wouldn't cut it for machine tool usage.
Some other ideas:
If you have nothing against wood and have a router you can glue. up sheet stock (plywood) into rather robust 90 degree corner brackets. Here you use a factor edge as a pattern for the router to create one good corner on the laminated stock. While this is wood you can get a rather robust block if you laminate enough material, use a good glue and maybe a few screws. The idea here is to exploit the factory edge which is generally pretty square.
Get a welding shop / fabrication shop to weld up what you need. Just make sure they have the machining capability to square up the bracket. Frankly this gets to be expensive but is a good solution for critical locations in your machine.
Buy a good cut off saw or table saw and a high quality square and DIY the materials you need. The saw is an additional expense but might be justified if you need to do material prep once the router is built. Note that aluminum can easily be cut on wood working class machines. However doing steel on these machines is a much tougher nut to crack. Only you know what your long term needs are so this might not fit into your plans.
Look for structural framing brackets. These can be for aluminum extrusions and in more general designs made of steel. Make sure they are robust with diagonal support. For example:
Display Part P2484W. Unistrut or equivalent is available at electrical and plumbing supply houses. This is industrial quality stuff so the prices are a bit stiff compared to your run of the mill home center crap. Also 90 degrees might not be that precise however they are generally made with substantial materials and paired up might be suitable for your needs. Angle brackets of these types though are not the most robust solution used alone. The Aluminum extrusion brackets might actually be a better choice if they have the gusset and are suitably wide.
At the moment I'm drawing a blank as to other off the shelf robust 90 degree connector methods. Maybe some other ideas are out there.