Are there any good places to fight the ability/racial mods for different races, such as kobolds, goblins, and whatnot?
Yeah, MM tells you a lot, also that Savage Species book is pretty good.
I was wondering about physical traits such as lifespan and such, which the monster manual usually does not have.
I'll bet someone has written all that up, somewhere. Ask this question on the WotC forums. You'll probably get 300 responses and start a flame war between gamer geeks who would argue that a Gryphon would outlive a Pegasus.
Some of this information can be pulled out of the old 'Ecology of...' series from Dragon magazine. If you have that compilation CD they did you could find a lot of it.. and then update to 3.5
Kobolds are listed in Races of the Dragon. Not sure about the others. Orcs you could extrapolate from half orcs, I guess. Hobgoblins probably live longer than humans and goblins probably live shorter.
No lifespan data, but there's a PDF that develops most of the rest, as well as developing a psychology for goblinoids, kobolds, lizardfolk, orcs, troglodytes, and ogres:
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)
As previously mentioned, the Races of bookes have lots of info on exotic races. THe Races of Destiny has half-orcs, the Races of Dragons has kobolds, and the Races of Eberron has TONS of information on Hobgoblins, goblins, orcs, and such from Eberron, so I can imagine that they would live about the same as any other of those species.
Or, as has been previously stated, make them up. Specifically, make them up to suit you campaign world. Official sources might have kobolds living only 35 years or so, but what if the kobolds in you game world actually had a lifespan closer to that of Elves? Six-hundred-year old Kobold Sorcerers, anyone?