Well, you're (mostly) in luck! This guide on iFixit has a step-by-step, location-based guide to what size the screws are, and where they go.
1) Other than the pentalobe screws for the bottom dock connector (which you can get here and replace with the iPhone liberation kit), it looks like the rest of the screws in the iPhone 4s are standard Phillips screws.
If you still have all your screws, though(and it sounds like you do, even if they're not all in your phone), you can compare them against the guide and see which goes where.
If you are dead set on getting a new set, it looks like you can get them from amazon here or here, though I've never used those supplier and can't speak to how good the screws are.
2) The guide I linked to above will show you what screws go where. But, if you're looking for a RIDICULOUSLY useful all-in-one solution, I personally recommend this: it's a magnetic screw mat shows you where your screws should go on your iPhone 4s, and will hold your screws in place while you reassemble your phone. It's super-awesome, and helps avoid situations where you have no idea where the screws came from :)