What you probably want is a voltage-controlled oscillator or VCO. There are lots of vendors that sell them.
Many VCO's will have logic level (square wave) outputs --- you'll need to look for one specifically with a sine wave output.
Also, 300-500 MHz is nearly an octave tuning range. You may have to look through a lot of parts before you find one that can cover this range.
Alternately, if you're designing for a reasonable volume product or have a budget to back you up, most vendors of frequency control parts are very happy to do custom products. The main drawback is there's typically something like 16-24 week lead time to get one made custom.
If you are just doing a one-off, or can't afford the wait time, there are various ways to build your own from standard parts.