I've used
https://www.itunescarddelivery.com/. They email a PDF with the code within about 10 minutes of ordering.
Though I did have one issue. I paid via PayPal, and my PayPal email didn't match the account email I created on their site. So I had a little back and forth convincing them that I wasn't a scammer. So just make sure your details match (if paying via PayPal).
They often have sales and coupons where cards can be bought at par or better. E.g. $50 iTunes card for 50 USD.