The first kind of department store you can find is the high end kind, where luxury products are sold. The most famous high end department store here in Italy is La Rinascente. It is present in the biggest and most touristy towns around Italy. Here you can find luxury brand make up and skin care products, such as Tom Ford or The Prairie, but also Giorgio Armani, Dior, Chanel, YSL, ecc. Expect the prices to be higher and the MUA to be snottier that usual! Usually the beauty and fragrance section is located on the ground floor.
For more popular prices and high road/drugstore brands, you may try going to Oviesse (or OVS). Oviesse is present in big and medium sized towns across Italy. You can find typical drugstore make up brands, such as Maybelline, or even cheaper, such as Essence.
Half way between laRinascente and OVS, we have Coin, which mixes high end and rare brands with drugstore brands. It is present in big and medium sized towns in Italy and in big cities in eastern Europe and Asia.
Especially in OVS and Coin the counters (or stands) may not have the full range of a brand, and stands for cheap and popular brands such as Essence may be sometimes completely empty. Be aware that in these store there may be sale assistants paid directly from a brand to promote specific items or to push their brand. Promoters sometimes may be the only personnel available on the floor and you may end up asking them for help. They will help you but they'll also try to sell you their products! Don't let them push you into buying from their brands if you are not convinced!
Hope it is useful for you guys!