If I recall correctly, Aziraphale does buy his clothes. Heaven began restricting his major miracles, so he buys good quality and keeps them in good condition, possibly with the occasional minor miracle. (How else would he have known that miraculous cleaning would leave him "always knowing the stain was there"?)
Aziraphale needs to keep his clothing spotless. It's a primary metaphor of his existence. Angels must be spotlessly sinless. If he removes a spot, it means he knew it was there, and that's awfully similar to having allowed it to happen through a lack of vigilance. If it appears and vanishes without his doing anything, then it wasn't his fault. (The way we know Lady Macbeth is having pangs of guilt is that she is so obsessed with removing the evidence: "Out, damned spot!" The way some other people insist they are innocent of something is to say they didn't know it was happening; if they knew it was happening, then they are guilty of a coverup.)
We saw him miracle money, in 1601 at the Globe theater. (They tell me there's a little "magic happening" ting in the soundtrack for it, so it isn't sleight of hand.)
Making money happen was easier when it was all metal and there was no need to mess around with serial numbers on bank notes (that would be counterfeiting, while making another genuine coin is just increasing the general wealth by a small amount). He may even have taken the metals from undiscovered mines instead of creating it out of raw essence. The increasing reliance on bank notes following their invention may even be the real reason for the book shop. He does sell books occasionally, and he charges very high prices. He may have stashed genuine first editions of books he thought might gain value in miraculously hidden caves all over the place. He sells second editions quite happily (presumably because he knows he has a first edition for his own collection).
All those statues of angels in the shop... in the 19th century they could have been another source of income. There was another fad for angel-decorated stuff in the late 20th century, which could explain why they are still present, even disregarding his tendency to have souvenirs and reminders. (It may also be another way of covering up his existence. If someone says "Angel!" and a customer hears it, it can be explained away as someone reacting to a statue.)