My Winix HEPA filter started blinking its little red 'check filter' light. When I bought the unit I'd gotten a spare set of replacement filters.
I go through how to recognize the need to replace the filters, I show how it's done, and below I have a link to an affiliate site where one can buy them.

Here's an Amazon link to the Winix I got. https://amzn.to/39twvBr

Here's an Amazon link to replacement filters: https://amzn.to/3fpqidq