I use a mix, and I got it off the internet like bowman said.
1 bag of sweetlix cattle mineral....$15.00...50lbs
1 bag loose white salt....$5.40...50lbs
1 bag di-calcium phosphate...$14.00...50lbs

I mix 4 or 5lbs at a time, and put it out once in early spring and again mid-summer
will last YEARS