I use https://www.amazon.com/gp/product/B08TQLHCFJ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 but when I bought them in Aug they were $16.73. $19.95 now but it's (5) packs. I also change mine once a week whether they look dirty or not. Sometimes the sounds are cleared afterwards even though didn't look dirty & they were only 21 cents each