Get a microphone and make your own. Its quite fun. There are great sound editing programs that you can use, like goldwave, to play around with the sounds.
Let me just say this,
There are many different aspects of being well rounded in the Half-Life mapping world:
Mapping
Moddeling
MODing
Coding
Sound
Texturing
Plotting (writing story-lines.)
and many more...
Its great to be well rounded with each. Its a good idea to get to know the basics of each of them, then you can choose your path, and follow it.