do you mean something like this?
http://www.homefilters.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=HF&Category_Code=5-6inch

these are media filters that fit into existing return air filter grill rack.
as there is more area of filter, these work well, and are not
restrictive like a 1" pleated filter is.
1" pleated filters are very restrictive.

this is a picture of the filter I use to show better what
is in the link I included.

best of luck

I am finding a filter for inside of my furnace. The slot is one inch

I just realized the offset in that filter allows a 4" filter to fit in most applications that originally were designed for a 1" filter, or appears to. If so this is awesome. I was lucky and only had to remove some tabs in my filter drawer for a 4" to fit and it makes a huge difference. I still keep the merv down so it isn't too restrictive though.