IMHO, the most important thing is to make sure you have the same LED's in all of your orange buttons for consistency (and have the resistors all the same, if possible with your wiring setup, etc) - otherwise you'll get different shades of orange in the different orange buttons.
I would prefer to have all my oranges the same color, then have one or two slightly different than the rest.
As for where to buy them, there's lots of places, everyone's inventory is always changing though, I got all of mine from a seller on eBay for really cheap.
As for the question of "how" to light them though, I prefer to have my LED's shining perpendicularly to the wall of the button. As opposed to shining up from the bottom. Less chance of going blind

and then the angle of your LED doesn't have to be as wide since the light gets dispersed pretty well throughout the plastic of the button. Though, if you do this, you'll need to plan on two led's per light, otherwise the light will appear stronger on one side, etc.
However, it seems from what I've read, that there are some easier 'pre-made' solutions for mounting LED's if you're willing to risk your eyesight and light them straight up from the bottom. So, you should get good results either way. For me, using all clear buttons, I got much better coloration by shining my lights in from the sides.