You can do an internet search or probably buy one at a local store. You have to watch the charge per minute and what other fees might be involved. They are basically a "virtual calling card". You call a central number from a predetermined number, have credit on your account/card and then call the country.

I just checked the one we use and it was 13 cents per minute in March. You can get started with as little as $10 in credit. $10 would give you about 75 minutes of calling time to SXM.

Below is the site I have used. Check it out and see if it works for you.
https://www.idirectdial.global/