Well can't be true. Something you told us is not valid.
USB is 5V and yet your round end is 9V. Hence there would have to be a buck-boost inside which is physically bigger than just a voltage drop. Hence either it is more than just a simple cable, or there's more information on a label somewhere.
Anyhow, what I can infer is the Wattage is very low but not so low it intersects all Anker products, so I can eliminate some less likely to be a good match. 9V-800ma is 7.2W so assuming 5W and say 20% power loss = 1.8A
Hence any of these will power you USB "simple adapter" to the round pin.
Powercore Slim 5000
Powercore 10000 as these are all at least 2A output
and then everything else higher in cost.
Less likely to be a good fit would be:
Powercore+ Mini as these are 1A output.
We must then to narrow the recommendation must work out how big an Anker powerbank you need. To do that can you let us know
- is there any more you can tell us like mah, Wh?
- how long does it take to recharge your device?
- how warm does it get when recharging?
- how long does your device last on a charge?
- how long would you like it to last?
From this I can guess within an accuracy of factor of 2.
Also, if you know make/model GIYF (we can search for information).