Testing a video transmitter for video bandwidth
It's not hard to make a oscillator of a few MHz.  In fact, I've done it by accident many times.

Use the VCO from a 74HC4046.  Supply the chip with power, and setup a pot to feed the VCO input.

If the VCO isn't a strong enough square wave, feed the output through a few stages of 74HC14 to square it up.

If you really need a phase lock with the video signal, you can phase lock this against the sync output of an LM1881.  Use phase detector II in the '4046 and add your own dividers according to how many pixels you want in a line.

With all this said, surely the vendor of the video transmitter has a spec for the video bandwidth it will support?  Also, NTSC kinda defines its own bandwidth requirements. From Wikipedia: