Ask HN: Historical Market Data for use in webapp?

I'm building a webapp that lets you practice trading. I don't really expect to make much/any money out of it and so I'm after a cheap source of daily historical market data that I'd be allowed to use in the app.

Lots of sites offer free historical data, but they appear to be for personal use only and I guess this doesn't qualify. Does anybody have experience with a supplier that lets you use the data in your own online app? (many thanks in advance!)