Hey Rafaelti,
By default, archived copy of downloaded configs are stored on your pollers in the following locations:
C:\ProgramData\SolarWinds\NCM\Config-Archive
You'll have a folder for each node which is managed by that poller, and in there will be the backup configs.
IMHO, the best way to manage your configs is using those already backed up by NCM, and if you need a newer version run a job to download it from the device via NCM and then take a copy of the config from the above location.