Are questions asking for a source to an artist's songs permitted on this site? For instance(this is an easy example), Where are the words to Yaakov Shwekey's song "Vehi Sheomdah" from? If they are allowed, what's the best way to ask them? Should I link the song in the question?
As I wrote elsewhere (not specifically about sourcing lyrics):
Questions about Judaism are on topic. Questions not about Judaism are not. Thus, for example, "Where does the tune for birkat hamazon come from?" and "Where did the sing-song chant that Jews often use while contrasting points come from?" are fine, whereas "Who wrote Aleh Katan by Avraham Fried?" and a question (now deleted) about the song "Yoya" have been closed as off-topic.
The same would apply to sourcing lyrics, in my opinion. If there's reason to think the lyrics are Judaism-sourced, then asking where they're a quotation from seems to be on-topic; I think you should include that reason in the question itself to serve as justification for asking.
Note, though, that you should definitely search the Web for your answer (including by searching for the lyrics) before posting the question to Mi Yodeya! On-topicness is not the only criterion for a good question; see more on this in the help center.