In our campaign, there are a variety of spell components and focuses that depend on what type of magic you do, etc. Most are fairly common, but may be a bit of a pain to come up with in a 'Modern' world. For instance, one of my spells requires an ounce of horsehair... but my guy is an Urban P.I., so I either have to find it or go to the 'magic shop' and buy it. One funny thing is that many of the spells I've learned require a focus of some type instead of a material component, and almost all are a piece of silver jewelry. So with several spells like that my character is decked out in all sorts of 'bling', without ever meaning to.

Now if the GM decides I have to get one of those 'grills' on my teeth like some rap stars have done for a spell, I may just have to draw the line.
Some spells are somewhat ritualistic in nature, and since our magic system often requires painful costs, they can get nasty. For example, one spell requires the use of <x> amount of wound points in blood to cast. You can take that nasty hit when you cast it, though spellcasters can 'bleed' themselves ahead of time and store it up; evil guys can just take it from someone else (if possible, in this case it had to be your blood, which the BBEG mixed into brownies and fed to some poor slobs to mind-control them).
And another note, you can read our Story Hour (in my sig) to see how a magic shop is handled in our world. In short, there is an 'underground' of magic shop owners that operate under a strict code of non-intervention. They will sell to anybody, for whatever purpose, and wont directly help you with any problem you may have with another spellcaster (other than sell you stuff you might need) and will not become involved in current events in any way. If you cross one (threaten, harm, rob, etc) they will all most likely find out about it, and that just would not be pleasant. They are often hidden in plain sight behind the guise of a rare book store, new age medicine shop, GNC, Spensers, etc. Ours is a New Age shop specializing in crystals, incense, etc. Except there is a large dreamcatcher hanging over a back door, and if you possess the ability to 'detect magic' there will be a code phrase that only you can read; speak the phrase and they will let you into the back to see the guy with the real goods. Magic items are interesting and most often not directly damaging/enhancing things (no +1 swords, armor, wand of magic missles, etc), so every trip is quite interesting. The costs are often pretty strange or downright nasty... one character traded a permanent (i.e. non-recoverable) loss of 1 point of Charisma for a specific item (and then a different character was ironically offered an elixer on a later trip that permanently increased Charisma by one point).