Scales to Play Over A7b9
Every chord is a subset of several scales: any scale that contains all of a chord's tones fits over it. Pick a chord to see the matching scales from its own root, the major keys the chord belongs to, and full fretboard diagrams for every scale on guitar.
A A7b9 chord contains the notes A – A# – C# – E – G. Any scale that includes every one of those 5 tones fits over it — from the root A, that means A Half Whole Diminished and A Spanish. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
A7b9 is not diatonic to any major key — it is a chromatic color chord, so treat it as a moment of tension: lean on its chord tones from the scales above, then resolve to the surrounding key when the progression moves on.