how i can install the ios without cf by using flash memory (usb drive ) >
By default, ISR G1 routers, including your 2801, do NOT support USB-boot of any kind. In order to support USB boot, the bootstrap needs to be upgraded and I seriously doubt if the seller did.
The only way to boot the IOS is through CF. You can get any third-party CF (non-Cisco branded) as long as they are supported and/or they are of the "standard" Type 2 CF format. It is recommended that you go to your favorite electronics or camera shop and get a few different sized CF from different manufacturer (size up to 8 Gb). Before you load the IOS into the CF (using your computer with a multi-media card reader) please ensure the CF is formatted using FAT 16. Not all manufacturers follow the Type 2 CF standard (and won't be mentioned in the pack) so you'll need to acquire a few different brand/sizes until one is recognized by the appliance.
