The good news is, HEPA filters are incredibly simple technology, and should be available locally from many home depot stores. We recommend you look for a HEPA filter with the following traits:
1. Bigger than the surface of your fan, so that no air leaks around the fan
2. Pleated, to provide a larger filtration surface
3. Has a rigid frame to ensure it remains in shape.
MERV filters – also sometimes called furnace filters – are also a good option for DIY purifiers. They have a lower filtration efficiency than HEPA filters, but meanwhile let more air through. Overall, they can give very good filtration performance. Read more: why are lower grade filters more effective at filtering the air.