I also don't have a local shop, so I get most off the internet. So you want all my secrets hmmmm. 
My regulars are
Tower Hobbies, good for your standard kits from most of the major manufacturers (Revell, Tamiya, Italeri etc). Interestingly although there are corporate ties to Revell, I can get most Revell kits cheaper from other venders, but the prices on the other brands are hard to beat. They frequently have specials too ($15 off $75 purchase etc) and the shipping is a fixed price so those big orders are nice, I got a box from them awhile back the size of my (then) 8 year old son and shipping was still only $9. They also have super fast service, sometimes only 2 days, and almost always less than a week.
http://www.towerhobbies.com/
Megahobby also gets a lot of my money, a little better selection of smaller brands than Tower, and the prices are close, Megahobby has been beating Tower on price for Revell kits recently.
http://www.megahobby.com/
For cars specifically, I will ofetn use Model Round Up, the ordering system is a bit archaic but the prices, selection and service are good. Primarily domestic brands though (Revell, AMT, Lindberg), also a good selection of resin and aftermarket.
http://www.modelroundup.com/
Sprue Bros has already been mentioned, I use them primarily for aircraft from many of the smaller manufacturers and limited run stuff, Eastern European brands in particular. They also have super fast shipping.
Squadron is further down on my list, but they usually get one or two large orders from me most years (over $100 = free shipping). Most of my model purchases from Squadron are added to book orders.
Ebay is another source, but I limit it to really good deals and hard to find stuff.
If you like resin I can give you a good sized list for resin car and truck stuff too.