Depends on where you're located. I buy most of my wigs online simply because there's a higher quality selection than you can find in most stores. Also, unless you're lucky enough to live close to a store that sells nice wigs, the cost of gas to drive to a store location is pretty much equal to or greater than any shipping costs you'll encounter.
I've bought a few wigs at Display & Costume. I don't know about other stores. What you'll want to do is check the fiber before you buy (most nicer bagged wigs come in bags that snap or zip shut, so you can touch the fiber). Rub it between your fingers. It should feel sleek and smooth. If it has a rough or "sticky" texture, walk away.
The style I've seen that's closest to Mousse's in stores is called something like "International Beauty", and yes, it has a woman on the front. The bangs are straight across and it comes to just below shoulder length.
For online shopping, try cosworx.com or amphigory.com. Both stock really nice quality wigs for reasonable prices. For something Mousse's length in a nicer fiber, you shouldn't pay more than $35 plus shipping. Cosworx ships really quickly, too.
For the glasses, I'd say either Harry Potter style (they're everywhere in costume shops now) or granny/hippie types, all of which will have round lenses. I've looked in thrift shops, which sell old glasses, and you can't find perfectly round anime glasses anywhere. The best you can do is costume lenses.
If you want the whited-out look Mousse has a lot of the time, I'd paint the lense with acrylic paint (craft or hobby shops, even in Wal Mart's craft department). Paint the inside, so the lenses will still shine. If you want to do the swirl Mousse has on his glasses, I'd paint it first in a different shade (I suggest light blue; black is too obvious and looks kind of odd). Let the swirl dry completely, cover with a coat of clear varnish or sealer, let dry, and then paint white over the top of the swirl.
Granny/hippie glasses are pretty cheap. The most I've paid is about $3.00, so you could get two pairs and leave one pair clean for Mousse's clearer moments (and so you can walk around wearing them and still be able to see). Also, one pair could be kept in a pocket in your sleeve, along with other small sundry items, to pull out as a gag for photos.
P.S. You rule. Nobody cosplays Mousse, and I will fangirl all over you when I see you. Yes, even if I'm cosplaying Shampoo at the time. Especially if I'm cosplaying Shampoo.
