Two different things

Disposable filters can be purchased at lowes, wal- mart, etc.

A dealer would need to wire and install a uv light

For info, look at ultravation. Just put it in your web browser and go to their site

UV light do work! We have had one in both of our last two houses. I would clean the ducts, and install a single tube UV light, there are some inexpensive ones on Ebay( less than 100) and they work great! We use Filtrete Ultra allergen filters, dont skimp on filters, use the best you can afford! But, dont pay for stupidity either... We are looking at installing a Trane or American Standard air cleaner now...

ARe there any independent case studies that show significant benefits to UV in ducts?

Uv lights have a purpose in a hvac system.

The air is moving far too fast for he uv light to have any effect in the actual airstream of a hvac system.

The real place for a uv light to be effective would be where the light can be directed onto the evaporator coil if one has a/c that is.

The evaporator coil drain pan must also be uv resistant otherwise you can cause problems to the drain pan in your coil.

be carfull about putting more restrictive filter into your filter rack than what you hae now without checking it out with contractor. You could do damage to your heat pump or a/c with a more restrictive filter
Come on over to the link I will provide and do a search for uv light or open a topic of discusssion and you will receive far more knowledge about uv light and different filters

Here is a link that might be useful: Hvac-talk.com

Filters are easy to find and UV lights do work - some have power sources that adapt to 115v making install a breeze. We like the Triatomic APCO product.