I will be joining a local police department and going throught the academy within the next 6 months. I also want to join the USCG as a reservist. What or how does the process work in order for me to become an Investigator in the USCGR? I know I must have 3 years of LE under my belt from the start.

You can stil join the CG reserve if you like, you'll just have to come in with a rating other than IV and do something else along the way other than CGIS related activities. After you have your 3+ years on the job, then you could try to lateral in rate to IV if there were any vacancies.