If you have a rooted device you can use a file explorer to navigate to the /system/app folder where they are stored after installation but if you want to access the apk it downloaded (ie to save it) you could (again I think you need root access) look in the data\apps folder which is where the market app saves them before installing.
However the already answered replies would be much easier to do
Sorry but I'm nomally 4 years behind the times. I just came across this. It appears, if anybody is having the same problem, that App Backup and Restore picks up the b4A app and will backup it up to its backup directory. I haven't tried to reinstall one but I can't see why it won't work, it seems like it might need a different Version number to recongise it as a new app. It doesn't need root,