6spdcoupe, 3.5max6spd, and PremierAudio are dealers on this forum, and will give you a good price.
For installation, you can install yourself, it isn't particularly difficult. But if you have to pay someone to install it for you, and it's coming out of that $500, then yes, you'll go over. If so, you could go with the Rainbow SLC 265 (about $100 cheaper) or the SLC 265 Kick (about $60 cheaper). That'll leave a little left over for installation.