Rough Riders are not available, and for the immediate future will not be. Here is an email response from Games Workshop regarding Rough Riders.
Questions about Rough Riders to GW
I wanted to share the following
email I received when I inquired about the lack of AM Rough Riders (at
least on the US site).
"Thank you for writing in to us, although I don't have good news for you.
I'm sorry but the Rough Riders kits are currently not available. It is
possible that they will return in the future as a plastic kit but we have
no confirmation or timeline for this.
Some existing kits we offer could be converted, such as Marauder Horesmen,
Empire Knightly Orders, or Empire Outriders.