I got a couple of these www.ebay.co.uk/itm/280840301713 - there are many sellers on Ebay with identical looking ones, so don't take that as an endorsement. The PSU is supposed to be 1A, but mine had 2A printed on them, but I bought a 4A one to go with it as I didn't trust them (they are very light weight adaptors).  It is a 2.1mm jack for the power on the hub.

I am in the process of moving everything onto a standard PC PSU, having made some Molex to 2.1mm jack adaptors last night. The 5V rail is 17A on my PSU, and I can plug my fan into the Molex plugs instead of having it tapped into my PC.

Bob

*edit: Don't forget to isolate the 5v pin between the host PC/RPi (and each hub if you use more than 1).  Some versions of the RPi have a resettable fuse built into the USB socket, but your RPi could try to draw its 700ma from the USB hub if you don't isolate it and don't have its power plugged in.  I use a 2-3mm wide strip of paper sellotaped onto the USB connector going inside to cover the 5v pin between each hub.  My external PSUs are different ratings, and I don't think they will balance the load if they are connected in parallel through the 2 hubs.  I can show a pic of the paper trick if you don't get it from my description.