First, welcome to StangNet!!!
I did a quick ebay search for your manual, but, as luck would have it, I found a 98 and a 2000 (along with every other year), but no 99. I'd suggest hunting deeper into ebay or 'googling' for one as well. (Googling meaning searching the internet with a search engine like
www.google.com)
Exhaust systems are as personal as color. The classic muslecar sound will come from the Flowmaster American Thunder catback. (I love mine.) Magnaflow is also very popular out here. Bassani is more expensive, but it is quality stuff. I love their catted X pipe...it's on my short list for next upgrades to my 02.
You can check the different sounds each exhaust combo will give you at
www.mustangexhaust.com - (By the way, registration is NO longer necessary to listen to the sound clips).
You can also search this forum for flowmaster, bassani, magnaflow, catback, etc - but be prepared for information overload - it's a popular topic.
Again, welcome and enjoy!