Finding Common Wire on Laars JVT boiler
Tap the bundle of yellow wires connected to the transformer

If you'll look at the wiring diagram for your system, you'll notice that your R terminal is connected via a red wire to the transformer via an inline fuse, and the other secondary terminal has yellow wires connected to it, as well as a green ground wire.  Furthermore, there is a crimped splice of several yellow wires hanging below the transformer in your photo -- cutting that splice off, stripping the ends, and splicing a jumper to it using a wirenut (I'd use 14AWG yellow stranded THHN for this, it's cheap and easy enough to get) will let you bring that common wire out via a jumper to where it needs to be.