Diaphragm 1st stage regs you like

[greeniguana]

If you want a more budget-friendly 1st stage, I've been very happy with my Dive Rite XT1. That was my first regulator and I still use it (albeit these days all of my XT1s are now on stage bottles and on a sidemount setup). It's super durable and service kits are readily available.

And they have a replaceable volcano....

 

[runsongas]

apeks DST is the standard. parts availability is better for the apeks style offshoots if you want to DIY service (eg HOG/dive rite/deep 6).

scubapro mk11/mk17/mk19 will probably be supported but the US pricing and parts availability are the pits. zeagle is a bit better priced, but parts availability is not good.

the SEA should be supported for near forever though (it shares nearly everything with the conshelf/titan kit except for one extra 014 inlet oring iirc) and trident sells the diaphragm and HP seat in the event OEM kits don't continue.

 

[Divin'Papaw]

Aqua Lung is my go to. Legend & Mikron. I’ve also owned Apeks & Dive Rite. Those regs are gone but I still use the ALs. These days they’re all I use.

 

[Mr. Sunday]

Apeks DST with the 5th port or the Scubapro MK19 would be my choices.

 

[lowwall]

Mares 22/32/52. Proven high performance regs with a very stable IP and replaceable orifice.

These use the same service kit that is around $20 and includes a new seat. No matter how old they are you can upgrade to the latest spec with a $25 upgrade kit (new orifice, seat and spring) and a $10 basic o-ring service kit. Kits are readily available from European shops that will ship to the US and occasionally on eBay and Amazon. Even if you can't get kits, the seat, diaphragm and o-rings are all standard sizes available from multiple sources and the orifice is used on all current Mares diaphragm firsts so will be available for a long, long time.

 

[Still Kicking]

Apeks DST, DS4, XTX200 etc.

Aqualung Conshelf/Titan

Aqualung Core (now Helix)

All workhorse regs.

 

[Fibonacci]

Not exactly what the OP specified, but I would look around for a good second hand Poseidon XStream MK3 and get it serviced... would still come in close to some of the other new options listed. Look for one with the integrated OPV, which gives you more freedom with adding a range of second stages incl Poseidon later.
They are an extremely high performance, very rugged and reliable first stage regulator (rated to 200m Norsok U-101) and under ice without the need for environmental sealing, very straightforward to service and parts are readily available.
Maybe I'm biased, owning two sets

First Stage Xstream Deep G5/8

Air, Nitrox, Trimix and HelioxDIN G5/8

www.poseidon.com

 

[Belzelbub]

Name some good regulators that use diaphragm 1st stages
current models that parts are readily available for and probably will be for a long time.

This is a good way to look at it. Parts that will likely be available for a long time.

All the regs in my family are Zeagle. All are diaphragm firsts. Currently have a ~2003ish DS-IV/V, a ~2014ish Flathead 7, and a ~2019ish Onyx II. Despite the first two being no longer produced, I have no problems getting them serviced. A big part of the reason is that apart from the Razor (direct tank thread for Pony, etc.) all the Zeagle 1st stages use the same service kits.

Everyone is currently having supply chain issues on some parts, but that's temporary. More concerning would be a reg that is no longer being made that uses a unique service kit. Likelihood is much higher that parts may be hard to come by.

 

[Jake 10]

Scubapro MK17 EVO
why?., quality, sealed, can be used in cold and warm waters

 

[tursiops]

Not exactly what the OP specified,

What an understatement!