If by cheap you meant smaller/lower denominations, then I would personally vouch for "Bitrefill" and "CoinsBee" ^{[both of them offer $5 Google Play gift cards]}

• In case you were looking for discounted prices, then there's "CardCash's Discounted Google Play Cards".
  ^{- Take note: I prefer to not vouch for them.}
• Other options: Gift cards providers