I find that I'm always in my highest gear. What options to I have to re-distribute the ratios to a more useful range
To get higher gearing, you'll need either a chainring (front gear) with more teeth or a sprocket with fewer teeth. Mountain bikes usually have smaller chainrings because off-road speeds are generally slower. For example, a mountain bike with a triple crankset with 24, 34, and 42-teeth chainrings would be limited in its top speed by the largest (42-teeth) chainring. In constrast, the largest chainring on a road bike may have 50 or 53 teeth, giving 19% or 26% higher gearing respectively.

Unfortunately, upgrading probably won't be as cheap or easy as you hope. You can't just replace a single chainring, so you would need to get a whole new crankset (the crank arm plus chainrings) and also a new front derailleur. Depending on how much your bike is worth, it may be better to get a new bike.