Hi there
I'm new on here - this is my first post.
I too have had trouble finding a suitable Tec 9 for the costume, but i got there in the end!
Rather then going down the "de-activated" weapon route, there is an alternative.
Back in the mid 1980's, toy companies made "replica" dart guns and water pistols.
Here's the Larami Tec 9 dart gun....
It's practically a perfect copy
There is also the Entertech Water Hawk - similiar to the Tec 9 but not as good...
(this picture has the water "clip" missing and also has the "don't shoot me officer - it's a toy gun" red cap)
They are both pretty rare, but worth keeping an eye out for.
Hope this helps
Morlar