They should be starting to spawn this time of year. They'll head up into mid-sized, rocky creeks, or into weedy shallows of lakes. You can sometimes catch adults using rod and reel with a piece of nylon rope that has been frayed to form a mass of loose strands; pull it past the gar and he will snap at it, entangling his teeth.
Otherwise you can wait until June-July when there will be lots of young in the aforementioned areas. Dipnet up a few and grow them out in a rubbermaid tub with plenty of small live prey (daphnia to start, moving up to glassworms, mosquito larvae, guppy or other fry, etc.) until you think they're big enough to do well in your pond. Be sure your pond has a healthy population of prey fish before you try to keep gar in it.
As always, be sure to follow your state laws.