Symmetric voltage oscillator
The simplest way to get such "inverted" clock signals is to get a "differential" oscillator that has both "CLK+" and "CLK-" output (clock and complementary clock), such as the
50 MHz Silicon Labs 530EB50M0000DG, the Cypress CY24293 (requires an external 25 MHz crystal to generate up to 100 MHz), etc.

If you already have a single-ended clock signal, you could generate clock and complementary clock output signals using a variety of IC with differential outputs, such as the ON Semiconductor MC100LVEL16, the Fairchild Semiconductor FIN1108MTD, etc.

Then, if necessary, you could use a DC blocking capacitor and a resistor (or diode) to level-shift (DC restoration) the signals where you want them, and perhaps a resistor divider to get the peak-to-peak levels you want.

It's possible to build a 50 MHz oscillator out of common parts based on a 74HC240 inverter, but it requires a bunch of tuning and tweaking.

At lower frequencies, I would generate the appropriate frequency, and then use an inverter to generated the complementary signal -- however, at 50 MHz, the inverter delays are a significant fraction of the complete cycle, so I'd prefer to use an IC with differential outputs so they're both delayed the same amount.
Out of curiosity, what sort of device are you feeding this kind of positive and negative complementary clock into?