Making your own copper sulfate solution of 1 mg contained copper per drop is very easy. Just measure out the blue copper sulfate crystals and the water. You can buy blue copper sulfate crystals at amazon or any large internet retailer. I look for 99% or 99.9% pure.
1 teaspoon copper sulfate crystals.
5 oz. of water.
Personally, I make a 2/5ths teaspoon into in a 2 oz. dropper bottle, as follows:
3 x 1/8 teaspoons, plus a quarter of a final 1/8th teaspoon.
3/8 = .37
.25 x 1/8 = .03
.37 + .03 = .4 = 40% = 2/5ths of a teaspoon.
Here is the math:
Source: Source https://en.wikipedia.org/wiki/Copper(II)_sulfate
copper sulfate pentahydrate density = 2.286 g/cm3
4.92892 cubic centimeters per teaspoon. source: google
4.93 cubic centimeters per teaspoon x 2.286 grams of copper sulfate pentahydrate per cubic centimeter = 11.3 grams per teaspoon = 11,300 mg per teaspoon of the blue copper sulfate pentahydrate crystals.
In its blue, hydrous form, it is 25.47% copper, 38.47% sulfate (12.82% sulfur) and 36.06% water by mass. Source, Wikipedia, above.
11,300 mg copper sulfate per teaspoon x 25.47% = 2878 mg contained copper per teaspoon.
There are 591.5 drops of water per ounce. x 5 = 2957 drops in 5 oz. water.
= .97 mg per drop
Further sources and information: