United States single phase 240v with 14-30r outlet

I want to use an emonTx to monitor my server rack.
I’m located in the United States.
The server PDU runs off a single phase 240volt 30amp circuit. The outlet is type 14-30r.
What kind of power supply do I need?
I was considering using the UK type provided by the store and wiring in the female side of an extension cord from the UK…
There has to be a better, less kludgy way…

Is your 240 V supply still 3-wire 120 - 0 - 120 V? (centre tap of the final transformer is earthed and the neutral).

Although the UK 240 V supply is 2-wire with an earthed neutral, we do have double-pole switched sockets, which would be safe on your 120 - 0 - 120 V wiring, and which would accept the UK a.c. adapter directly.
(One of these mounted in/on one of these.)
If you ask nicely, the shop might be able to include those for the appropriate number of “special payments”.

What I don’t know is how your local rules would view that - I’d be afraid they might demolish the whole building out of fear of the unknown, even though from what I see of US practice, our rules are significantly stricter than yours (i.e. we don’t allow wires to be twisted together with a thimble on top, nor do we trap wires under screw heads, all connections are made with clamping screws in terminals). In the UK, those sockets are normally protected by a 32 A MCB, so effectively exactly the same as your supply.

Alternatively, and still assuming your supply is 120 - 0 - 120 V, and if you’re concerned about measuring half of the supply with the regular US adapter, you could use two of them, one on each half, then wire the outputs in series to give you 24 V, and then add a 120 kΩ series resistor to drop the voltage seen by the emonTx back down to 12 V (yes, it is 12 V on no-load, 9 V on full load). You could only use that for sampling the voltage, you must use a separate 120 V to 5 V d.c. USB adapter to power the emonTx.

A final alternative would be any 240 V transformer in a box, it would need to have a nominal 9 V output, giving around 11 - 12 V on no-load, and then you could use that to power the emonTx as well as sampling the voltage - but I couldn’t give you any hints whatsoever as to what the phase error of your transformer might be, so you could have problems calibrating it.