It's been 8 years since I bought my cartons so I don't remember exactly who I brought from. However, I ran a quick search and came up with these two vendors whose prices are reasonable. I prefer a recycled pulp carton that will compost nicely. You can see that you can buy all kinds of elaborate cartons, but do you want to look "commercial?"
I don't. I bought the custom stamp from Staples. They make gorgeous business cards, too, should you ever need any.
Regarding stamping the boxes: put a piece of wood or something to brace the back of the carton top. That way, you'll get a nice clean stamp and be sure if you buy the jumbo cartons that are more form fitted and have the holes down the center, that your stamp is narrow enough to fit on one side. Knowing what I know now, I'd wait until the cartons arrive and then take a couple down to Staples and let them figure it out. They were incredibly helpful.