If you are more worried about the frequency accuracy than purity of the sine wave then a quick way would be to buy a 100MHz CMOS crystal oscillator (couple of dollars/pounds/euros). They are available in 4 pin DIP packages (power, ground and output).
It will output a square wave in theory but in practice the output driver will not be fast enough to produce the 3rd, 5th and above harmonics so it will be pretty sine wave like anyway. To improve check the output impedance from the datasheet and then choose a capacitor value to make a low pass filter (cap between the output and ground) with a corner frequency of about 80MHz. That will attenuate the 100MHz signal a bit but will get rid of most of the 200MHz and above frequency components. On a scope it will look like a sinewave, you might see small harmonics if you FFT it but for many applications its not an issue.