The common practice around these parts is to add 20%, but there is no standard amount. Having said that, I don't ever mark something up; it just goes straight through to the client. I like it better when I'm being compensated solely for design work, and I've never felt comfortable marking stuff up. Perhaps that stems from having spent too much time at the other end of things, managing a budget.
The way I see it, if I buy stock photography or code for a slideshow, I'm not adding value just because I bought it, so I don't add to its price, either. Research time is either built into the original quote or (occasionally) billed at an hourly rate.
Where the purchase is printing, I bill (or factor into the project quotation) the time I spend working with prepress or doing press checks, but even in cases where I've picked the printer and made all the arrangements I will usually have the print house make their financial arrangements with the client directly, rather than being a middleman.
For other expense items, I bill the client the expense immediately and separately from the project fee if it's more than a few hundred dollars, in which case, depending on the client, I may wait until I'm paid before proceeding with the purchase. These terms are always clearly laid out in the contract.
The only thing I consistently add are finance charges, such as when the payment is by credit card.