You haven't indicated which bibliography style you intend to use. Is that because you haven't decided yet which style, or styles, you'll employ?
BibTeX has been around for well more than two decades. The original BibTeX style files --
apalike, and a couple more -- all date back to an era when online resources didn't exist and thus weren't going to be cited. Unsurprisingly, these style files don't accommodate online material all that well. If, for some reason, you must use one of the original BibTeX style files, you're going to have to follow the path you've already identified, viz., use the
note field of various entry types to store the relevant URL strings.
If you can use the natbib citation management package, you should use that package's associated style files --
alphanat -- instead of
alpha. Being of a much more recent vintage, the
natbib style files do know what to do with fields named
Still working with BibTeX, the apacite package and its associated style file (also called
apacite) offer even more entry types and fields types for each entry, many of them related to online and other electronic features. Do check out table 3 of the package's user guide for detailed information on which fields are recognized by which entries. E.g., you'll find that
urldate fields are recognized by all of the package's entry types (with the exception of the
literal type, with which I'm not familiar).
For the ultimate (at least for now) in terms of power and flexibility when it comes to bibliography creation and management, do consider switching to the biblatex package. It's still in development, so things may at times not be as utterly stable as they are in the BibTeX world. (One of the downsides of BibTeX, some would say, is its utter stability -- and immobility.) Unfortunately, because
biblatex is still relatively new, many journals and publishing houses that accept LaTeX documents as inputs will not run
biblatex. Thus, it may be a good idea to incorporate at least some backward compatibility in your bib files, so that you can generate, if need be, bibliographies using either BibTeX or