You can't normally purchase on a non-connected PC, but you can use the scenery and activate it, using the offline activation method.
If you have FS9 or FSX on a computer that has internet access, you can purchase the scenery from there, without activating it, by using the "Buy on Web" option. This will just launch a browser, so you'll get an email receipt with your serial number, that you can activate on the non-connected PC, with the offline activation.
Or, if you don't have FS9 or FSX on the connected maching, you can purchase from HERE
, which is the same address that would have been accessed by the browser, when launched from FS9.