We're currently capturing lead source automatically through either a source parameter in the referring URL, or through Smart Campaigns to determine other lead sources (like if they come from organic search). So we're able to capture lead source for anonymous leads this way as well.
Because of this, we're not asking for the lead to provide us with their lead source in the form. We want to get the true lead source that brought them to the site to ask for a quote, and asking them may not give us the most accurate picture of that.
Also, a lead will sync to SF in one of two ways: they fill out a quote form, or their lead score gets to the threshold we've set for it to sync.
So, I want to pull the most recent lead source from our running lead sources (the custom field we created) at the time they come to site and convert into a lead - whether they fill out a quote form, or their lead score is high enough that they are synced to SF without filling out a quote form.