How do I obtain a copy of my PCA certificate?

You should have gotten a license from the state if you took the Certification Exam and the place that gave the exam submitted it to the state.
Contact the State License Board for the state where you took the exam.
Most times you will have to show proof of CEU's and probably pay to have the license renewed. This is probably done 1 time a year (some might be valid for 2 or more years depending on the certification. not sure on that)
If you were employed your employer or former employer should also have a copy

PCA is a Personal Care Assistant. Just FYI.

In my State the Nursing Board gives certification. Where did you take your course? Maybe that place has a copy.