Dictionary for Vietnamese to Chinese characters
Wiktionary lists the vietnamese pronunciations for every chinese character they have info on (at the bottom of every page under the "Vietnamese" heading).

Other than that, the Nom Foundation database is good for converting vietnamese words to chu nom by sound, though you'll need to know enough characters to know what you're picking, which you also gave reference to yourself. It's a good resource, but limiting if you're trying to input into a computer.

Not all Chu nom are in unicode, though, so finding them can be challenging. The Nom foundation gets around it by using images, and is probably your best resource, but isn't as detailed on things such as meaning and usage (for which wiktionary has an advantage).