My advice would be to use either the Premium Data (DukasCopy) or Meta-trader Demo (JFD) data in EA Studio, if possible. If the asset you want to generate strategies with is not listed, then I would look at DukasCopy to export historical data from. From my experience, they have the most amount of bars. On most M15 time frames, for example, you should get about 65 000+ bars. Just remember to change the Symbol Settings to match the broker you’ll be trading with.