Mine get Yellow Rockies in their pens, if you know that you are in a copper deficient area you can use the Red Rockies.
You get them in the horsey section at ther merchants.
I also use other suppplements, like garlic, seaweed powder and caprivite, about once a week durning winter, more Caprivite when milking in mid-summer.
Never heard of bicarbonate of soda being used. Wouldn't that just make them burp, and no useful minerals in that one. If you look at US/australian websites they may mention Dolomite, this is a natural form of ((I think and it's too late the cross-check) and Magnesium and Calcium Carbonate. Also contained in Caprivite.