Blogspot doesn’t like to let me comment for some reason, but I felt a need to talk about this. You can still get some decent stuff from Radio Shack, so by all means. Most locations have been more of a cell phone store than a general electronics store for the longest time, but they’ve kept a decent collection of components.
As for the replacement? They’ve been all around for a while, and I actually doubt that many makers have been using them as a source of supply for a while. Most decent sized places have a store like Gateway, and most of these stores actually are better for the electronics enthusiast that Radio Shack. Some even still carry vacuum tubes. You might have to do some digging to find your local store, but it’s worth it.
The other alternative is online, of course. sites like Sparkfun or Solarbotics. These sites focus on smaller stuff(though still useful, like Arduino), so a site like The Robot Marketplace for the heavy duty stuff. AndyMark will be familiar with those who participate in FIRST Robotics, as they’re a sponsor and a main source for the competition legal parts.
As I said, likely if you do a lot of this stuff, you already know of the local sources, and there are even more websites out there. I’ll be happy to make a list in a post if I get a lot of links in the comments.