Best option is to add a booster pump as last guy suggested. Another tank can be installed closer to the garden, but you will not see significant pressure boost. It will provide more water storage at the point of use, but not a big boost in pressure.
Grundfos, Pentair, Goulds all make good irrigation boosters like you could use. You might find some cheap makes (like Wayne, or Flotec) at a hardware store that will work too.
You will need a 115V single phase power supply to run a booster pump, probably 15-20amps of available power off your service panel.