I have always found that the black crickets yield better results than the brown ones. My best explanation for this is that because of their dark colour and bigger size, they provide better contrast in dirty water.
To catch / collect them try the following. In one corner of your garden, set up a spot where you dump some crass clippings, and soft garden waste (leaves etc), and put a piece of shade cloth or similar material over this. Keep this pile moist but not soggy wet. Soon you will get some crickets moving in. You can catch them by hand by lifting up the tarp, or you can bury a small bucket in the pile so that the rim is at the level of the surface of the plant materials. Now put a piece of bread in the bucket, and leave it overnight. Next morning you should have some crickets trapped in the bucket.