Few Taishanese-English dictionaries exist, none of which exist online. The closest thing you’ll find to an online dictionary is Hoisan Sanctuary. This website hosts a short word list of some 40 words covering primarily family terms and function words (e.g. pronouns, question words, etc.). The site also provides a brief English-oriented sketch of the sound system with a partial orthography and discussion of tones.
Beyond the linguistic description, you will also find the Hoisan Sanctuary Forums, which foster language-oriented discussions on Taishanese. Most of the enthusiasts who frequent the forums are individuals of Taishanese heritage who are seeking to explore and renew the bond with their home language. Their enthusiasm is remarkable. Most also lack a background in linguistics, and few appear to speak Taishanese as a dominant or primary language.
The forum discussions shed light on common attitudes and experiences shared by many overseas Taishanese. For example, contributors often share words that have been seared into their memory since childhood. (These are often rather colorful expressions.) But some also report back on individual research and resources they’ve identified. For example, it was through the Hoisan Sanctuary Forums that I learned about the DLI coursebook’s audio file supplements at Ben’s Cantonese Practice Journal.