Mortar is mortar. It's not cement although it is related. It's what you use to make a brick wall. It can be found in most any big box store. The purpose of the mortar is to provide a solid base under the tub so that it does not flex. I say a solid base, but that does not mean you want solid mortar under the tub. You place piles around the area where the tub will set then set the tub in place. The weight of the tub will squish the mortar out. If you could see under the tub, you'd see there would be voids in the coverage, but there will be plenty of support. I'd be sure to get mortar placed around the drain area especially. You don't want any flex there for sure.