I can help with analyzing microscopic plastic particles, but not microplastics. Let's start with most expensive and go to least expensive.
For R&D, we used a liquid nitrogen cooled microscope FTIR with ATR accessory. The ATR might not be needed. It did pretty well, although black samples could give problems. Cost, with libraries, was about $100 k. It would easily handle particles smaller than 5 mm.
You might be able to get by with a standard diamond cell FTIR. These are much less expensive, but still would run in the tens of thousands of dollars. The 5 mm range particle size is something you would want to try before buying.
Polarized light microscopy is another techique. Less expensive, but it takes a fair amount of operator expertese. Don't know if it works on all plastics.
This will give you some places to start investigating. Please come back with any questions.