If a card says, "discover a clue at your location" does the clue come from the reserve or from the location? For example, could you use that card to gain a clue at a location even if there were no remaining clues?

I'm pretty sure the way it reads is "A clue at your location" so it would be one on the location card.

Yes, investigators can only ever get clues off of locations. They have no way of generating clues from the bank on their own.

Text on a card ability along the lines of "discover a clue at your location" absolutely requires there to be enough clues available to pick up on a location. This is of particular importance to cards like Roland's unique weakness Cover Up (to remove clues from this card, you must first go through all proper steps that would otherwise allow removal of clues from a location), the survivor card "Look What I Found!" and Mystic card Drawn To The Flame (you can only get two clues if two are available), etc.

Essentially, clue quantities are by default fixed and finite game resources which cannot be pulled from the common token pool. They get added to the board when a location's revealed and that's your lot. There are exceptions to this is the Core Set campaign, but those instances have to be regarded as exceptions, examples of 'when a card ability overrides the rules text, the card ability takes precedence'. The exceptions I'm aware of are a location in scenario 2 which allows to players to pay something to generate 'extra' clues that come directly from the common pool, and a location in the third scenario which repeatedly refreshes its clues every turn.