Use bac water if the HGH is going to sit in your fridge for other injections.
There was a shortage in the past but there isn't now, you can get bac water from CMS (sponsor) if you want to order online.
Sterile water as previously mentioned will not keep the HGH safe for use for long periods. For example Sero 6 mg vials gets 18 IU's of bac water and if you take 2 IU's a day you need bac water to keep it safe for the 9 days.
Maybe you would be OK with sterile water but I wouldn't chance it. As @DeviousD said, you can ask your pharmacist to order it in if they don't have any in stock.