Am I able to upload art I've commissioned and source both the art that's been uploaded to my profile as well as a link to the artist's profile? The artist did not post it themselves so I'm unable to directly link the source to a post from the artist.

Yes. Any valid source is acceptable. Obviously, sources from the artist are ideal, but sometimes, sources can also come from a commissioner or other legitimate receiver of art, even if there are sources from the artist. Sometimes, that's because the artist doesn't post the art, the commissioner posts a better version than the artist does, or maybe it's just an alternate, but equal source.