If you use ready-made adventures you run the risk that they hardly give you a new type of plot. It is mostly background information, interesting places etc. That is cool, obviously, but they might just turn out to be one adventure more you gotta have to get through...
(This was not supposed to sound rude or anything...)
However, I would really like to recommend you to take a look at the
British Islands Travel (BIT) supplements "101". I love the concept. They got
"101 Cargos",
"...Governments",
"...Travellers",
"...Religions",
"...Rendezvous" (Bar Encounters),
"...Patrons" (The kind of guys, an adventure usually starts with),
"...Corporations",
"...Lifeforms",
"...Starcrews" and the most generic but greatest
"101 PLOTS". Maybe some of them have been published by/for GURPS.
Traveller Classic Supplement 6, 76 Patrons is something similar, or even the original idea.
Another great source for plot ideas is
Lisa J. Steele's "Mysteries" for GURPS 4th Edition. She explains how to run a Crime adventure... She gave me great insights on what to do and what to avoid, mentioning the short term memory of the players and stressing to keep things simple. One of her referrals is S.S. Van Dine. He wrote 20 rules for writing detective stories. Though the book is directed to Crime Adventures, it is an awesome source for every GM. (Just do not tell your players about those tricks.)
A-aaand another generic but gracious source would be
S. John Ross' "The Big List of RPG Plots".
In general, I like to get inspired by the material for GURPS and Traveller.
Well, well...