Nsgarch, any inexpensive tone generator used for musical instruments would be fine-- also I googled and found a few free online websites under "free tone generator" or audio frequency generator".
With most generators, you can choose the pitch, so if the music is in key of D, then you can select your generator to D. It may take a bit getting used to (musicians are more familiar with pitch recognition), but with a little practice, you can match the pitch of the tone generator (any frequency) to the key or pitch of the recording. It really only takes a few seconds when you get used to it. When your skills improve, you can tune to a tone generated "A" even though the music is in key of D. Most of the notes in a musical passage are "centered" around the key. Hope this helps!
With most generators, you can choose the pitch, so if the music is in key of D, then you can select your generator to D. It may take a bit getting used to (musicians are more familiar with pitch recognition), but with a little practice, you can match the pitch of the tone generator (any frequency) to the key or pitch of the recording. It really only takes a few seconds when you get used to it. When your skills improve, you can tune to a tone generated "A" even though the music is in key of D. Most of the notes in a musical passage are "centered" around the key. Hope this helps!