Crutchfield or Parts-Express are most likely your best bet.

If I remember correctly, the Monsoon system is a little different from typical systems and I think the speakers are only 2 ohms.

If you get regular 4 ohm speakers, they will work, they just wont be as loud at the same volume level if using the factory stereo head unit.

I changed all of the speakers out in my previous car, a '98 TA, and later changed the head unit.

The factory door speakers can get pricey, you may find them on eBay, but other sizes will fit. Check the forums on LS1.com, very good info there.