Buying A Dodge 318: New Or Used
Like I said, Dodge has been making the 318 since the late 1960's, and its design has remain relatively unchanged through the present day. Sure you've got new technologies surrounding it, such as fuel injection systems replacing older carburetors and computer controlled ignition systems replacing the old points distributors; but the internal mechanical components of the engine are nearly the same from year to year. Even the physical dimensions and motor mount locations are identical!
Why do I bring this up? Because you might be able to find a used Dodge 318 for a fraction of the price of a new motor, and then "retrofit" it with the appropriate parts of your model and year. For instance, if your late model Jeep Cherokee needs a new engine, you might be able to find a 20 year old engine that would actually work under the hood (with a few modifications and updates, of course). It's not always cheaper to go this direction, especially if you're not doing the work yourself, but it is an option.
On the new front, there are several manufacturers who sell Dodge 318 Crate Engines. These are effectively brand new or at least professionally reconditioned motors, and will run like new - sometimes even better than new. Just be aware that each company offers different packages, so while one may offer the entire engine from top to bottom (block and internal components, intake manifold, carburetor/fuel injection system, exhaust manifold, heads and ignition system), others might only offer some of these parts. That's why you can't go off price alone when comparison shopping for crate motors.
Crate engines are available through pretty much any local auto parts store, though you'll probably need to special order them. Or you can shop online at places like summitracing.com or any other major auto parts online retailer. And the crate engine builders themselves usually have their own websites with order forms as well. If that doesn't get you where you want, try the Dodge, Chrysler and Mopar home pages.
Where To Buy A Dodge 318 Engine
The nice thing about the Dodge 318 is that it is incredibly simple. So even if you don't have much experience working on engines, this is a great "starter motor" to learn on. Granted this is more true of the older engines than the newer, computerized motors with several additional pieces of emissions control equipment. In fact, if you're thinking about buying a motor to "tear apart and rebuild" for the sheer sake of learning, this is a great option.
Buy New Online
I'd start with the official Dodge website and go from there. My next stop would be the Mopar website (just so you know, "Mopar" is the performance division of Chrysler/Dodge), as they are more geared towards parts and even performance.
Then I'd browse the online search engines for terms like "Dodge 318 Crate Engine For Sale."
Buy New Offline
There aren't many places in the majority of cities that stock new engines, but auto parts stores are your best bet. They will probably need to order them in, so be prepared for a wait.
Buy Used Online
Because these engines have been around for so long, there are plenty of opportunities to find one online. You can start with online auction sites like eBay (eBay even has a "motors" division I believe), as you can often pick up some really good deals there. Then I'd find a site where people can sell their merchandise directly without much third-party interference; Amazon has such a system, where it's possible to list and sell your stuff with only a small surcharge added.
Then you can move to online want ads, like Craigslist. Craigslist is great because not only are people willing to part with their stuff for very cheap prices, you can also post a "wanted" ad. And you can do this for pretty much any geographical location in the United States even if you don't live there. For instance, you can search and post on the Boston Craigslist even if you live in San Francisco. Sure there will be some logistics to work out should you strike a deal, but it's an option I think merits some consideration.
Buy Used Offline
I love junkyards. Or "Salvage yards." Or "Wrecking yards." Or whatever the politically correct term for these businesses currently is. Regardless of what you call it, it's basically a car and truck morgue. They are full of wrecked vehicles, and customers are allowed to pick the functional pieces off of them.
Because the Dodge 318 is so common, it's hard NOT to find an engine in a junkyard!
But beware: You'll need some serious mechanic skills to make this fly. Not only will you need to remove the engine from the "host" yourself, but you'll also need to evaluate its functionality, often without being able to fire it up. This isn't a viable option for those who aren't comfortable working on cars or simple "weekend warriors" who consider changing the oil a major mechanic skill. But if you know what you are doing, you can truly get some amazing prices on great motors.
As long as you know where and how to look, it's not hard at all to find a Dodge 318 for sale.