He's right about posting in the D/P sub-section but in all fairness, it's not a tough question to answer and even if you post in that sub-section you'll just get told to post in the right THREAD anyway
You have to link up for a pretty good chunk of them. Check out GameFAQs.com and look through their FAQs or look at the map inside Nintendo's Ultimate National Pokedex guide (you'd have to buy it cuz you need to rip the map out). Both places have lists of every type and if/where you can find them. Mostly starters and legendary, yeah, but there are plenty that don't appear in the game. You could always trade for any but that might take a while and you don't know if you're getting some that people used Action Replay or whatever to doctor up. The GTS would help but people always expect you to trade a level 100 legendary for something like a level 4 Pidgey or whatever. Besides, you have to have them in your Pokedex to search them in the GTS.
If you don't own the old ones... Well, I bought FireRed and Emerald within a week just cuz I like the games enough and they're not too expensive. So it's not a big deal really.