Scales to Play Over F#7b9
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 F#7b9 chord contains the notes F# – G – A# – C# – E. Any scale that includes every one of those 5 tones fits over it — from the root F#, that means F# Half Whole Diminished and F# Spanish. Start with the first scale listed: it is the most common choice, and the others color the same chord tones differently.
F#7b9 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.