Built-in Flash of Canon SX10 or even in Canon SX50 is not powerful enough to do advance photography like taking macro photos. So you need to have a external flash with more power. Canon Speedlite 90EX Flash is not the best one when we consider the power but it's the lowest cost one out there and lot better than any cheap Chinese flash units out there. I have experince with those cheap Flash units and those work only when they want to. and After few month stop working.
Canon Speedlite 90EX can be bought from Amazon for around $70 and the next best one is Canon Speedlite 270EX II Flash (Here) which cost you around $150 with free shipping. But if you can go for that one then you should because it's lot better than the 9EX specially for night shooting.
Before buying an external flash better think why you need it and for what you need it. Canon Speedlite 90EX is good for small things like taking cool macro photos and for as control flash if you want to use a slave flash. But if you want to cover an indoor event like a party or wedding then you need a more powerful flash. Then you need to see which upper level Canon Speedlite flash fits to your budget. Check more Canon Speedlite flash units and compare prices (Here).