1 to 10-3 is not that difficult i.e. you are asking for a stability of 2-3 MHz at 2,5 GHz. Not that easy with a L/C oscillator, but a Hi-Q resionator should do it. Cavity might be one option, or a DRO (dielectric resonator), Make your own resonator with a low-loss coax (semi rigid) and a good multi turn capacitor (johanssen), and be very careful with temperature drift, you will be there.
Another way would be to start with a crystal oscillator and mulitply to what ever frequency you want. Draw back is spurious level and the amplifiers and filters needed.
Using a PLL would be quite simple and you can optimize the oscillator and loop filter for just one frequency.