My teacher is Karen Sharp, an awesome Jazz sax professional and she has me working on a number of exercises to help me improvise. She has also suggested I transcribe some of Dexter Gordon's solos and also melodies, carefully studying and mimicking the phrasing as well as the choice of notes. It's not easy but it is fun. Again once you have something you like try it in all the keys.
I am using
Transcribe to help this and also trying to play be ear - just small passages/riffs of one, two or four bars and then trying them in different keys.
Pete does have some great things on his main website as mentioned earlier. I do have
100 Ultimate Jazz Riffs For "Bb" instruments which is OK but to be honest I prefer learning direct from the masters. Having said that, i am starting to jot down any short passage of music I hear that I'd like to try as the basis for a riff.
Also, Karen has me working on chord tones to the 7th and 9th and it's amazing what great riffs you can play if you use jazz articulation and these 4 or 5 notes. I also sometimes try taking a tune - say Doxy or Autumn leaves and play the same rhythm and articulation but with different notes within the chord progression of a different song. Doesn't always work that well but does get you thinking!
Now if only I could do any of this improvisation stuff spontaneously and get my brain/fingers working quicker I'd be pretty good!