You can't raise only the calcium - the double positive charge on each calcium ion has to be balanced by two negative charges from some anion or anions. Thus you need calcium salt of something other than sulfuric or hydrochloric acid. Phosphate and lactate come to mind as the only practical ones. You are limited in how much phosphate you can add by the very low solubility of apatite (the phosphate of calcium) and by the flavor of other acid anions such as lactate or citrate.
As you can't buy calcium mono or dibasic phosphate or calcium lactate from your LHBS (but you can from Sigma or, in the case of the lactate from a health food store) it is probably easiest to add lime (Ca(OH)2) in amount sufficient to contribute the desired level of calcium and then add phosphoric, lactic, citric.... acid in the amount required to restore the pH to the desired level (presumably mash pH).