As per my understanding and experience, u do not need any specific 'subwoofer' cable. I suppose u can cut some speaker cables and use that one. Since its a splitter so u would need max half metre cable only.
And as far as availablity if these things is considered, these can be found at computer parts stores or online easily.
It is better to buy a professional subwoofer cable as the cables are proper shielded OFC ones.....putting connectors leads to unwanted humming and possible ground issues....I have used such split connectors in the past and currently use the same analog version for the set top box......