apt-show-source: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/apt-show-source - from package: apt-show-source

You may install the required package with command apt-get install {package}

Error in other languages:
apt-show-source: bevel nie gevind nie
apt-show-source: comandă negăsită
apt-show-source: comando não encontrado
apt-show-source: commande introuvable
apt-show-source: command not found
apt-show-source: käsku ei ole
apt-show-source: không tìm thấy lệnh
apt-show-source: komanda nerasta
apt-show-source: Kommando nicht gefunden.
apt-show-source: kommandot finns inte
apt-show-source: komut yok
apt-show-source: nie znaleziono polecenia
apt-show-source: níor aimsíodh an t-ordú
apt-show-source: no se encontró la orden
apt-show-source: no s'ha trobat l'ordre
apt-show-source: opdracht niet gevonden
apt-show-source: parancs nem található
apt-show-source: perintah tidak ditemukan
apt-show-source: príkaz nenájdený
apt-show-source: příkaz nenalezen
apt-show-source: команда не найдена
apt-show-source: командата не е открита
apt-show-source: 命令找不到
apt-show-source: コマンドが見つかりません