You mean copper fungicide? Most folks recommend you not wait until later. I start spraying my tomatoes when they're about a foot tall and spray every few weeks from there on out.

I get mine from Johnny's:

Here is a link that might be useful: Copper Fungicide

Copper works better me than Daconil. But it is a preventive for leaf diseases not a cure. Use early and often.

I wonder what people think is the best [most effective] fungicide for tomatoes.