I have experience with prepaid cards and found them to be more trouble than they are worth. For me the only ones that worked without calling the bank were prepaid cards usually only easily found in smaller preset denominations (25,50,100). These were cards that were actually sold and owned by international banks. So if you were to want to fund more than 100 you would have to buy multiple cards or find the large denominations which are usually only found online and then you would encounter the same problem trying to purchase these as you would funding the offshore. The loadable cards were usually all owned by domestic banks that would put blocks on these transactions much like a regular CC that you probably tried and could not get to go through.
It has been awhile since I have tried funding with a CC, I stopped because it was just a hassle. My method for needing to deposit with a CC would to be to call my CC (US Bank, Chase, Citicards would usually be the easiest) and tell them I was traveling internationally and needed the international block on the card removed. They would usually remove it for 30 days, after that you would need to call again. I know that this method used to work very easily with Bookmaker's processors once you got the block removed. You CC will probably charge a small international fee on each transaction.
Hope this helps. I have been using Western Union to make deposits. On a related question I wonder if anybody has had experience with getting red flagged by Western Union or Money Gram? I have heard stories that if you use these methods too frequently or for large amounts that your name could get red flagged, making it difficult to use these services. I have also noticed that Bookmaker seems to giving me lower limits to make the deposit. In the past I know I could send as much as 2500 by western union to bookmaker. This last time I made a deposit I noticed that they capped it at 950.00. I did not ask why since I was not going to send that much anyway but it got me wondering why they are doing this. I am sure they are not concerned about me getting red flagged by western union, but maybe they are making this change because of a larger problem they are encountering.
I do not keep as much money as I used to in off shore accounts and do not send large amounts of money anymore. It would just suck to have another option for funding taken away if I got red flagged so I am trying to avoid it.
