Trailer Lighting Using a Dynamo Hub
Velocity definitely make 305 rims, and at least when they were in Australia would use their available extrusions to make semi-custom rims to order - most often so you can get the number of spokes/drilling pattern you want, but they would also make odd sizes if the extrusion was compatible. But they've moved to the US and I have no idea what they're like now as far as custom builds go.

SJS in the USA also stock 305 rims and I found quite a lot of hits just using google so decent rims in that size seem to be fairly widely available.

Schwalbe make bicycle tubes in a wide range of sizes with both Schrader and Presta valves - if you can find what you want on this page your LBS will be able to get it (although it may take a while). There are quite a few choices in 305mm presta tubes.

You may have more problems getting a dynamo that's designed for a 305 wheel. The circumference is half that of a 622 (27") wheel so it spins twice as fast at the same road speed, and unlike a childs bike your trailer is going to be used at normal road speeds. SON make dynamos for folding bikes with small wheels so if the Sanyo doesn't work I suggest trying SON. I have one and like it, although I had to replace the bearings after about 50Mm.