For the past couple of years we've leveraged several different distributors/resources.
First try is Streakwave.....which is a top level reseller for UI. Of course we prefer this since we purchase below MSRP.
We used to sometimes use Ingram Micro (who...gets their stock from Streakwave)...but a few years ago we started shifting away from Ingram...aka "ChingGram"...since China bought them up.
We do purchase lots of stuff from D&H...and sometimes they have stock....also below MSRP so we try them early on.
Often we do purchase direct from store.ui.com when things are avail....
We also have used DoubleRadius...not in the past year or so, but...we have.
A common one is B&H Video, located in New York City...so regular shipping usually gets it to us the next day. They surprise us with stock at times nobody else has stock. When I went to a Ubiquiti training class in NYC a few years ago, there were 2 or 3 sales guys from B&H that went there to learn the product more....and one of them I would order bulk from and get a little discount. He's no longer there.

...so we pay MSRP from them but shipping is fairly low since they're so close.
Flytech Computers in Florida is another resource we've used a couple of times.
We try to keep pretty good stock on our shelves...ordering when stock is available rather than waiting for a project and hoping to find stock right then and there. I know this isn't do-able or doesn't make sense for many people..depends on your monthly cash flow, number of clients, etc.
Price wise...yeah, since we bought quite a bit of them at MSRP....sometimes having additional shipping costs on them, our out of pocket cost is quite high and over MSRP...so we do have to appropriately mark them up. Price of the "project"..and we tell our clients that..and tell them unfortunately with stock shortages we have to do that...and our clients get it...they understand.
I have been keeping my eye open for another brand to start using, which might have more stock. Thing is, we've been doing so much UI for...jeeze, since '06 or '07...we have a TON of networks out there. However, we may have to start using another brand.
Aruba has that "Instant On" product line which looks great, however...only switches and APs. No router. At least last I knew. And yes we're so spoiled by Ubiquitis Unifi controller....we want router/switches/APs to all be managed together like Unifi does.
I couple of other IT guys I know jumped over to Engenius...and I signed up for to be a reseller, just haven't ordered anything yet.
TP-Stink...I mean...TP-Link...came out with a Unifi clone/rip-off called Omada. I really don't like TP-Links products..have used them occasionally over time, and years ago took on a client that had their "managed access points/switch" setup...which was total crap..couldn't wait to replace that. BUT...a few IT guys have taken the chance and deployed a few Omada setups and they like it. I actually went to an RV campsite that wanted me to take over their maintenance....they have an Omada setup across their RV park, I looked at it for a bit...sure looks a lot like the Unifi controller...just subtle graphic changes. I passed on it.
On, and..Arista...who bought up Untangle....has a whole product line from router..switch...to AP. Management is coming together. Have not seen any units yet. The "north east" or "eastern" sales guy for Arista is located in Maine and called me back around Thanksgiving to say he's stop by our office to meet us, since we do (did) a lot of Untangle volume. But..haven't heard a peep from him...so....patuey!
So....yeah....we've been managing to do OK with Ubiquiti....right now I have a DM Pro, the 8 port aggregate switch, and 5x of the 48 port Pro's...with a buncha larger HD and InWalls ready to deploy next week at that new client I mentioned the odd 365 hack on yesterday.
If you're really screwed on getting them in time for your project, I have 3x on the shelf right now....but they'd be priced high. Not too far of a road trip..lol.