I buy the boxes at one of the discount supermarkets. They come in threes. Keep the receipt and use it for the next three A's or D's. They only allow a certain dollar amount for box receipts so you have to divide it by three and declare that amount for each shop.
Seal the boxes thoroughly by putting clear plastic tape over all of the paper surfaces. When the boxes are delivered to your house peel the labels off immediately. The longer they're on the box, the harder they are to take off.
When the boxes become so worn out that they require the post office to apply their own packing tape, start using them for the B (ReadyPost) shops.
I have boxes that have been used 15-20 times each.
PS: Just make sure your mail carrier leaves the boxes somewhere where they won't get wet before you can pick them up.
**************************************************************
One buzzard to another while circling high overhead (paraphrased), "Patience hell! I want to shop somewhere."