Consider a scenario where you would only need to download the rpm and not necessarily install it.
You will have to install an additional rpm called yum-downloadonly
yum install yum-downloadonly
Once the above rpm is installed all you have to do is run below command.
yum install sendmail-cf-8.13.8-2.el5 -y --downloadonly --downloaddir=/tmp
(In above example sendmail-cf-8.13.8-2.el5 rpm is downloaded and stored in /tmp .in your case it could be anything.)
The above command will only download the rpm and not necessarily installed it.
(I would recommend you to run above command where rpm getting downloaded is not already installed in my case it is sendmail-cf-8.13.8-2.el5 .Otherwise you will get an error saying rpm is already installed.You may remove the rpm using rpm -e )