Well, there are a few (have a look at the link for Danger International and Justice Inc. here under).
But mostly, I have used scenarios written for other games.
MSPE being a quite simple system, you generally just have to make some stats to replace those of the NPCs in the adventures and you are ready.
Here is a first batch of publishers of scenarios I have used, and they have much more.
I have linked to pdf products if available, for ease of access.
FGU Daredevils
Danger International & Justice Inc. (3rd Edition, but look also at 1st and 2nd for more).
Gangbusters and Weird Tales
Daring Tales of Adventure
D6 Indiana Jones You'll have to hunt them on Ebay or Amazon or any other place on the web, but those adventures are worth it.
Thrilling Tales
Call of Cthulhu
WWII Operation Whitebox
Battleforce Bravo Adventure Pack #1
GURPS WWII
Achtung Cthulhu
Daring Tales of Adventure
D6 Indiana Jones You'll have to hunt them on Ebay or Amazon or any other place on the web, but those adventures are worth it.
Thrilling Tales
Call of Cthulhu
WWII Operation Whitebox
Battleforce Bravo Adventure Pack #1
GURPS WWII
Achtung Cthulhu
Twilight 2000 whilst the setting seemed outdated a few years ago, we played some very brutal games at the time with heavy casualties. We just changed the rational for the advent of the apocalypse.
Classified I have not played them yet, but the scenarios seem interesting...
Modus Operandi site about espionnage RPGs (including MSPE); You'll find informations about Spycraft, Top Secret and Top Secret SI which have produced lots of scenarios that can still be found at reasonable prices.