Fabric.com has some gorgeous buttons and a nice selection. The one pictured here are made of shell with a laser cut design--they make this plain white shirt look downright classy and sophisticated.
If you like the Celtic style, there are plenty of options, too. These are a real favorite of mine, also available at Fabric.com. They are solid metal, rather than hollow, so they're not suitable for lightweight silks, but they're just fine for most fabrics. I put a set of these on a men's tropical wool vest.
Placement and size of buttons is important. Oversized buttons rarely look good on a button-down blouse--you're better off using 1/2" or 5/8" unless the shirt is specifically designed to accommodate something else. Buttons should be sewn at the center front mark on the pattern--resist the urge to move them in order to make a shirt fit more snugly, it'll only pull the shoulders in and make the shirt drape oddly.
Shirt buttons are not difficult to sew, though many people think so. With doubled thread, tie a knot in the end and take 5 stitches for buttons with one set of buttonholes or 4 stitches in each set of buttonholes for buttons with two sets. Some people advocate making a thread shank under the button, but I've never found this necessary for shirt buttons--coat, vest, skirt, or trouser buttons, though, that's a different story. You do have to sew them loosely or the fabric will get puckery and twisty and you put the buttons through the buttonholes. Some people sew over a toothpick or a straight pin; I like to use the shaft of a machine needle.
Shank buttons on a blouse or shirt follow the same rule: 5 stitches with doubled thread will hold the button secure.
Buttonholes are just as important as buttons, and a machine buttonholer is a must--unless you love hand embroidery and want to learn how to make hand-sewn buttonholes. They are beautiful when executed perfectly and can make your garment look very, very high-end, but done poorly, you might as well do them in bright red acrylic yarn and scream, "Look! I made it myself! Badly!" Expect to practice a lot first.
Machine buttonholes, on the other hand, are usually a breeze--unless you have a sewing machine famous for doing lousy buttonholes. Most machines make acceptable buttonholes, though, and chances are good that yours is one of them. Tension can be tricky with machine buttonholes, though. Remember that you want the buttonhole to look its best on the right side, so sometimes it's prudent to tighten the bobbin tension just enough so that the needle thread is pulled to the underside just a little.