@import should look for any .css.scss files located in your assets load path, be it in app/assets/stylesheets, lib/assets/stylesheets, vendor/assets/stylesheets, or any additional paths added by gems, so it's odd that you're getting an error.
Note you do not need to pass a folder name when using @import unless the file is in a subdirectory of assets/stylesheets - sass-rails will look through every folder on your load path and see if the resource exists relative to those folders.
To ensure that lib/assets/stylesheets is in your assets load path (which it should be, since it's a default option) crack open a Rails prompt and type Rails.application.config.assets.paths. As an example, here's the result for one of my applications:
ruby-1.9.3-p0 :012 > Rails.application.config.assets.paths
=> ["/Users/tom/ruby/qa/app/assets/images",
"/Users/tom/ruby/qa/app/assets/javascripts",
"/Users/tom/ruby/qa/app/assets/stylesheets",
"/Users/tom/ruby/qa/lib/assets/stylesheets",
"/Users/tom/ruby/qa/vendor/assets/javascripts",
"/Users/tom/ruby/qa/vendor/assets/stylesheets",
"/Users/tom/.rvm/gems/ruby-1.9.3-p0/gems/jquery-rails-1.0.14/vendor/assets/javascripts",
"/Users/tom/.rvm/gems/ruby-1.9.3-p0/gems/bootstrap-sass-1.4.0/vendor/assets/javascripts",
"/Users/tom/.rvm/gems/ruby-1.9.3-p0/gems/bootstrap-sass-1.4.0/vendor/assets/stylesheets"]
It's also worth ensuring that sass-rails is up to date and running the latest version, since I believe early versions had limit support in terms of cross-folder @import.
N.B: I also believe that convention suggests files such as Bootstrap or jQuery should go in your vendor/assets folder rather than lib/assets