Bringing in plants from outside of your country requires an import permit and a phytosanitary certificate (a paper certifying that the plants are insect and disease free), making the purchase of growing plants from abroad quite laborious and expensive (you have to pay for your permits and certificates). However, there are few to no restrictions on importing seeds from other countries. Just place an order online and pay by credit card or Paypal or go the old-fashioned way and send in your order and payment by mail. The seeds will be delivered to you wherever you live, even if that is at the other end of the world.
There are a few exceptions, of course. It may be forbidden to import the seeds of specific plants locally considered weeds into some countries or states. Australia, for example, has severe restrictions on what seeds you can import. California also has many restrictions. Most readers of this blog will however find there are no restrictions on seed imports to their locality.
Click here for one quite extensive list of catalogs offering seeds and plants. Note though that there are many, many more sources of seeds all over the world.
Have fun ordering seeds this year!