Need a C connection at my Nest but boiler controls have no common

You're not missing anything. There are a number of auxiliary power supplies on the market -- including, I believe, one by Nest itself.

That said, you will find that the Nest is quite possibly the worst choice for controlling a steam heating system you could possibly make. They can be made to do a so-so job of it, but you must disable all of its learning features and set it for -- I believe -- true radiant. It was never designed for steam -- or, or that matter, hot water or radiant -- and never intended for it.

Good luck.

@theONEendONLY

If you know a little electrical you can use your Nest. My nephew put a nest in for his steam boiler and struggled with it for a year with no C wire. It's an easy fix. You need a 120/24 volt 40va transformer and a Rib #RIBu1C relay. You can get both of those at Supply House.com

Mount the transformer somewhere where you can get 120volt power easily. You can get it from the boiler circuit or any other circuit. So hook 120 v to the transformer Black-Black, white-white.

Mount the rib relay on the same junction box you mount the transformer on.

You need 1 3 wire cable run from this junction box location to the Nest.

Run a 2 wire cable to your boiler from the same location.

1. On the common side of the transformer connect the C wire from the Nest & the white/yellow common wire from the Rib relay.

2. On the others side of the transformer run a wire to RH on the Nest (use the red wire)

3. Run the W (white wire) wire from the Nest to the Rib relay and connect to the white/blue wire

4. Now you just have the 2 wire cable left.
connect 1 wire to the Rib Relay common contact and 1 wire to the Rib relay Normally open contact.
So 1 wire goes to yellow on the Rib and the other wire goes to orange on the Rib.

Over at the boiler disconnect your old T stat wire and connect this new wire. It doesnt matter which terminal get which wire ....doesn't matter.

Then your done

@theONEendONLY -- you enquire as to what makes the Nest so bad. Like so many things, for what it's designed for it's not bad at all. Overpriced yes, but it's actually very good. For forced air heat. Forced air heat responds to thermostat changes very quickly, or at least feels as though it does, and the Nest is designed and intended to vary the temperature in the space in response to its use. It does this by sensing room occupancy and also the occupants desires.

That's wonderful. Problem is, most other heating systems don't respond that fast (the extreme being radiant, which may take days -- but ;even an oversized steam system can take a couple of hours to change the temperature by 5 degrees) and the poor Nest keeps trying to raise and lower the temperature as people come and go -- and both it (trying to "learn" behaviour) and the heating system trying to hit a constantly moving target get confused, for lack of a better word. So they wind up fighting each other -- and using far more energy than they should.

The Nest does have settings to disable all the learning and occupancy figures, and to limit or eliminate setbacks, and to understand, if you will, the much slower response characteristics of anything but forced air. They don't come programmed that way, though, so the homeowner or installer has to do it -- and usually doesn't, or can't figure out how (the instructions are, to put it charitably, opaque).

I personally have other problems with it, not particularly wanting the Google and the rest of the internet to know my usage patterns and occupancy, but that's a different issue.