James,
At $4 a shell, I admire your desire to even try it. If you could shoot lead my recommendations would be entirely different, but my advice, as regarding loads, would be to buy whatever is the cheapest in the appropriate shot size, which has already been covered well here. If 12 ga. is the cheapest, use that. 1 oz. of steel #7s should do fine, doves are not very tenacious of life. But, good patterns are important, as the birds are small.
I would agree with other decoy recommendations, and would add that, if you have the opportunity to shoot in fields with center pivot irrigation systems, climb up a tower and strap your decoys up on top of the pipeline. There is no better way to use dove decoys than that. Just be careful not to shoot the farmer's system in your excitement to get on a darting and jiving dove.
If you live in a dry environment (I know less about Ontario than you probably do about Georgia) a watering hole will be a hot spot in the late afternoon. Sounds like you may be going this alone, so the low bird cautions are not as critical for you as those of us who shoot in groups with others in a field. No different from shooting a flushing quail or pheasant.
One bit of actual shooting advice for incomers. Don't wait too long to pull the trigger. A dove coming straight over you is moving faster than he appears and if you wait until it looks perfect to shoot you will probably miss him as he will be passing you and doing all sorts of evasive maneuvers as he does. The lead will also go from nothing to several feet in just a second. Shoot him just a hair before you think he is in range and you will dump him just fine. He will have covered more of the distance between you and him than you think, in between the time it takes you to decide to shoot and the shot actually gets there. This will allow you to take a double, if the opportunity is there, before the second one gets past you.
Have fun, SRH
