Do most biologists accept the theory of a single common ancestor of all living things?

Does anyone have any ideas on where that common ancestor came from?

Yes to both questions.

It is not impossible that life arose multiple times on earth, however scientific concensus is basically overwhelming in its support for a single common ancestor for all life we know about. See for example the wiki page for a bit of background.

As to where the common ancestor came from, or perhaps (what I think you mean) how did life arise in the first place? The origin of life is referred to as "abiogenesis". There's plenty of hypotheses but this is not really my area so I won't comment on their relative merits. If you search this site for "abiogenesis" (box at top right of the screen) you'll find a lot of answers to similar questions - starting with this one … hp?id=9419