If you have the factory fog lights then the 510's fit perfectly. If you don't, then you have the bumper inserts with holes too small for the them to fit there. You can order the other grey plastic inserts, but to install them you have to remove the entire bumper.
I've had the 510's in the factory holes, but to be honest I don't think they're all that great. They throw off a good wide sweeping pattern approx. 15' out in front. I drive with them on all the time at night because the factory headlights are so pathetic. They don't cause other oncoming vehicles to flash me, but if they were mounted up higher on the bumper, they'd likely be too bright for oncoming. Also, the PIAA's run hot, and I've had to replace three bulbs over the past four years... . not cheap, and a PITA to change out.
Places like Summit Racing would have best pricing, and even then, they're pricey compared to some other brands.