I often made my own fixtures using marine and auto bulbs. Sometimes I found really nice lanterns at import stores. I made nice redwood ones. You need to calculate the load of the bulbs you buy to make sure you get a suitable transformer.
I have also been known to buy the kits from places like Big Lots. The plastic fixtures themselves do not hold up especially well and look generically cheesy to me but the wire and transformer with light sensor or timer contained in the kits is worth the chimp change charged. You can raid the sockets and bulbs and stick them in something else too.
Lighting stores will have the best pre-made low voltage fixtures but they may be out of your budget range. If you must shop at a box store, just stay away from the plastic fixtures as they dull pretty quickly and get brittle fast when exposed to the elements. They break easily when you need to change bulbs too.
Main thing to do is sketch out and scale your yard to paper. Then make yourself a diagram of what you are trying to light and how. As mentioned, determine the load of the circuit or circuits and get an adequate transformer.
If twinkle lights are in your plans think about the newer LED ones. I guess look to see what LEDs are available for more conventional fixtures too. Last I looked, they were still fairly pricey and not too bright.