I always get preservative free & sterile ones! I use the Refresh brand. :)

Refresh Plus Lubricant Eye Drops, 50 Single-Use Containers, 0.01 Fl Oz (0.4mL) Each Sterile https://www.amazon.com/dp/B000052XFC/ref=cm_sw_r_cp_api_fabc_CuDUFb3Z45ANX?psc=1