Bitmain are probably the only reliable site. But their delivery times are questionable. You can check the hardware sub forum here on the forum and use an escrow to assure everything goes smooth.
Bitmain was the only good reliable site in past but now for me it don't worths to order anything from their website. Their support sucks, they hang on this forum last months too, their shipment is bad so ordering from them is a headache.
You can use aliexpress, alibaba, ebay. Prices are high because of antminer s9's ROI and current bitcoin's price. In past you could pay very, very cheap price but bitcoin's rise is a result of such expensivity.