More often than not, I re-use the original hardware. I buy a lot of antiques so the existing hardware is usually very unique and perfectly fit for the piece. I get most of my pulls from Hobby Lobby. They have a very large variety and great prices. If you don't have one near you, they sell them online.
I've also been known to get some from Lowe's.
If the piece has missing or just plain ugly pulls I opt for now ones. Or I do a mix of old and new.
The top are new and the bottom drawers are original
Mix. I love to keep them if the piece comes with circular pulls.
New, these bin pulls go with just about anything.
Bin label pulls from Harware Hut
On the pieces below I used the original pulls
A couple of tips for choosing new hardware for a piece, is to take one of your already painted drawers with you to the store.
That way you can try on each piece until you find the right fit.
Also, keep with the style of the dresser. Modern pulls on an antique piece might look unnatural.
I hope this helps!