Where do you live? You can google “pilates certification” and see which organizations have certification classes in your area. Make sure to learn in depth about the particular organization’s principles and certification guidelines, and pick one that fits what you are looking for. It also depends on what equipment you want to become certified in. There are some out there that are quick certs where you really don’t learn all the ins and outs of what Joseph Pilates has created. On the other hand, some certs take 2 years to complete, and others take weeks or months. There are some certs that are more “modernized” in that they are based off of the classic exercises but infused with newer stuff, and then there are some that strictly teach the classic way as Joseph Pilates created.
Also, look for workshops that will compliment the Pilates Mat cert you currently have (ie using other props, or progressing the exercises to more challenging ways).