We can't give out names of sites on here, but I would suggest searching for kratom. Sites pop up, most sites don't take credit cards because the government is cracking down. They want to ban it, but it actually helps people. Anyway, if you know how to use bitcoins, you're good. You can also do COD, which is where you get a money order and give it to the delivery person. Do your research. Whatever site you find, research it first to see what others say about it, just to make sure the product is good. You might come across 1 or 2 sites that still take credit cards, but like I said, it's mostly bitcoin and COD.
You can get it in pills or you can get the powder, buy some 00 sized empty capsules (I prefer vegetable capsules, not gelatin) and fill them yourself. It's easier that way so you don't taste it. Powder is usually cheaper too. With 00 sized capsules, it's about 0.5 mg in each capsule so at least you can tell how much you're taking. It's also best to get a scale, just to be sure. You can start with at least 3 to 4 grams and see how you feel. Good luck!
Forgot to mention, you can also check around your local tobacco shops like Wild Bill's and see if they have any in stock. That way, you can easily go get it.