From what I've found looking around the web, you need to check with the customs or quarantine department to see whether you need a Phytosanitary Certificate and/or an import permit. Apparently each country is different as far as these requirements go. If you need the certificate to import the plants, the place sending you the plants has to have a quarantine officer inspect your order of plants and sign the Phytosanitary Certificate before it ships. Apparently you have to handle this on your end of things. I haven't seen anything on how much these services cost, but if I come across them I'll let you know. I hope that was somehwat helpful to you.
I didn't even think to ask....I was just assuming you're in the U.S. If that's the case, this is what they had on the U.S. Customs site:
6. Plant and Plant Products. The importation of plants and plant products is subject to regulations of the Department of Agriculture and may be restricted or prohibited. Plants and plant products include fruits, vegetables, plants, nursery stock, bulbs, roots, seeds, certain fibers including cotton and broomcorn, cut flowers, sugarcane, certain cereals, elm logs, and elm lumber with bark attached. Import permits are required. Further information should be obtained from APHIS. Also, certain endangered species of plants may be prohibited or require permits or certificates. The Food and Drug Administration also regulates plant and plant products, particularly fruits and vegetables.
Still not real specific, but a place to start anyway. If you want to poke around the site, maybe you'll find better information:
You will need to apply for an import permit and the green/yellow stickers that come with the permit. Check the USDA website for more info.
The grower overseas can supply everything else like the phytosanitary certificate etc. Importing plants can be quite expensive as the phytosanitary certificate and shipping(by box for small plants) can total $30-40 on the average.