As a general rule, the version of the ADT plugins in Eclipse are paired to a specific SDK version. It's not very well documented, but if you update one at any point, you need to update the other. The version number of the tools and the plugins are usually in sync, but in some cases, they aren't. In fact, right now (June 27) the SDK is version 19 and the plugins are 18. Of course, that will change later today when Google IO starts. The intent is to keep the version numbers in sync, but it doesn't always work out that way.
You can get the older versions of ADT and/or the SDK from the Android tools site. In some cases, you have to figure out the secret for the URL to directly download the older packages, but they are there.
It shouldn't be necessary though. The plugins and the SDK work with all prior versions of the platform, so you should always be safe installing the latest SDK and plugins directly from the Android Developers site.