The other day I was looking for an iOS app that could access and download files form an Amazon S3 bucket. The only real one I could find was Transmit, which cost $10 and is being removed from the store. I search more, but finding no others, I paid the $10 and bought it even though it likely will not be maintained. It did the job and probably will do so for a while until Amazon changes their API.
Unfortunately, I did not see this article on Macstories about alternatives to Transmit and in particular iFiles, which is cheaper and may do the job just as well.