I'd start looking here: www.yarndex.com to find a yarn that has a similar guage. It may recommend a different needle size (but will probably be between 7 - 10), but you mostly want to match the stitches per inch.
As to how many you need to buy - you need to look at the original yarn suggested. Say it it coems in 500 yd sken and they want you to get 5 - you need 500 x 5 = 2,500 yards. Now look at the yarn you decide to use - say that comes in skeins of 650 yards. So take the total 2,500 yards, divided by 650 = 3.85 or 4 skeins.