One of the top miners in terms of hashing power is the Antminer S9. These miners have become very popular and are very high in demand. To order some from the source is hard to make happen as they are selling out fast. At the time of this post bitmain is sold out of the S9. In case you didn't know, bitmain is the manufacturer of the S9.
So this leaves a few remaining options to get the S9:
However, even with those sites it is likely you will only find pre-orders for shipments on later dates. Take a look around and you might find an S9, but it will be at a higher price due to the high demand and limited supply.
Part of the reason for the limited supply is that big mining farms are making deals with the manufacturers to get there hands on the S9's. This makes it harder to buy just one as there are less on the market for sale.
Another good option to take a look at is Avalons from Canaan and miners at balkai. And aside from owner the hardware yourself, you can get good rates on hash power with HashFlare. I have a bunch of hashing power with HashFlare and post about my cloud mining profits here.