I bought through "For Books" (it's apparently not JUST "for" books, it's just the name of the company?) and I'm in the US. People apparently can buy DLsite points through Amazon jp, but it takes some effort to get a JP account set up and get the address entered correctly.
FB is so easy it's just making a login and adding your normal payment info like any other site.
Also, make sure you read the amount properly when buying points off For Books, I undershot how much I needed by 50 cents only to find out you can only buy points once per day and I had to wait, hoping I could still make it in time before the new-account discount ends.