There are a couple of sources of instructions and year catalogues on the internet, but all the small leaflets that follow the sets? Is there any source/inventory of these?

What do you mean by “small leaflets?” Do you have an example?

You can check out lets build it again. There are hundreds of older Lego sets in their archive that might be of some help! 

Small leaflets, like the ones that come in polybags or small promotional kits? 

Do you mean the single sheet folded set promos that they had in small sets, or the stapled ~10 page mini-catalogs that used to be included from time to time (or both)?

The booklets are often referred to as "Medium-size" catalogs (vs. the "Large" or seasonal "Shop at Home" full catalogs that were mailed out), and Brickset has a number of them in their scan archive like this one from 1988:

As for the folded, single-page ones, I've seen those calls "Small Catalog (Sheet)" or "Mini Catalog", and Peeron/PICSL has a few of them (and a lot more of the Medium ones):

http://www.peeron.com/cgi-bin/invcgis/scans/?ct=1

 

I mean the folded single sheets, in the early eighties often only showing city or classic space. In the mid-80s, there was single sheet folder divided into 6 squares a side, showing all themes. City, space, castle, technic...