I would say that this springs into mind if you are looking for adventures in specific places or at specific levels.
Many of the adventures from Dungeon should work for this (check online for lists). They are usually set in generic lands and not too long. You would have to look for the 3.0/3.5 material.
There were books for the second edition titled "The book of lairs" which described fleshed-out encounters usable as fillers or random encounters. I believe they showcased a monster per entry, with some notes how to expand the encounter. Since you can find it for free, might be worth a look.
Someone made a conversion of Secret of Bonehill for 3E, which you could use or cannibalize (easily found by googling). I am not sure, but there might be others like that around.
On ENWorld you can find the "D&D PDF Collector's Guide". There are lists of old adventures and magazine issues, which can be easily converted, cannibalized or directly used. Most of them are not free, but links to online stores, so ou may have to dig around to find what you want (for free).
Adventureaweek.com has modules for Pathfinder/5E/3.5 which look good. You have to subscribe to the site ($10/month), but if you continuously need material, that should be OK. There is one free adventure, which you can check out for their style and quality.
On adnddownloads.com you can find short and free adventures/encounters, a couple of which are for Pathfinder, others for older D&D editions (The Fall of Fairhaven would be level 4-5).
Although you would have to move the setting just a bit, Feast of Ravensmoor is awesome. Creepy as heck and fits the level.