I have about 30 Grain of wheat street lights. Should I use dc to supply them? What is recommended for a power supply?
Incandescent light bulbs will work on both DC and AC. Depending on their voltage rating, you could use the fixed AC terminals of an old power pack, or the variable DC output, depending on whether you want to be able to adjust the brightness of the lights.
With that many bulbs, be sure your power source has adequate output and doesn’t become overloaded. Some really cheap toy train set transformers probably can’t handle that much of a load.
For safety, you should probably regard each bulb as drawing at least 30ma of current. Thirty bulbs at 0.030ma each would be 0.9 Amps. When you add electrical resistance of the wire, plus the possibility that some bulbs may draw more than 30ma, I would use at least a 2 Amp power supply.
If you don’t have a spare power pack, go to Wal-mart or similar store and get a 12 Volt plug-in wall-wart transformer with a power output of at least 2 Amps and use that.
Quick and dirty way to figure out the amp draw is to take a multimeter to the light bulb and measure its resistance. take their rated voltage and divide it by your measured resistance and you will get the approximate amperage draw.
Example:
12V bulb measures out to have 400 Ohms resistance. 12V/400R = 0.03A or 30 mA.
That is of course going to have the bulbs running at 100% brightness. If you wish to lower the brightness simply use a lower voltage power supply (9V versus 12V) or use resistors in series with the light bulbs. If you are worried about overloading your power pack, then you can, again, use resistors in series with the bulbs to lower the amp draw.
Claymore,
Unfortunately that method won’t work with lamps and bulbs as their cold resistance is much lower than their hot resistance. (remember the old tail light bulb as a current limiter?)
I suggest you put an amp meter in series with the gow bulb and measure the current directly. then multiply by the number of bulbs. And the bulbs will last 10 times longer if you drop the voltage a little below their rated voltage.
On the question of AC or DC, AC will prevent any electrolysis effects (corrosion) but both work OK on bulbs.
Bah works just fine, but thats why I said approximate! 
The only problem with using an ammeter inline (which is the most desireable way of measuring amperage) is that if he doesnt know what the ratings on the bulb are, he doesn’t know what value resistance to put in! Chicken and the Egg senario. There is always experimentation with various values of resistors, which will have to be done anyways, but grabbing the cold resistance of a bulb is the best starting point.
What WOULD be interesting to watch: after calculating the value for the required inline resistor using the cold resistance of the bulb, hook up an ammeter inline with this system and watch current over time (say t+10 mins).
As current goes up over time you can use ohm’s law and the max current recorded to recalculate for a new resistance value.