You can buy crushed coral at your local hardware store. Basically a bag of crushed shell. Mix this in with your choice of sand ( pool filter sand with black moon sand turns out nice ). This is fairly cheap and will buffer your water to where you'd be fine. Chemical additives are difficult to manage and can cause rapid swings in your pH which is worse than having a steady, albeit not ideal pH.
A higher hardness level ( which crushed coral/ limestone, etc. ), will help keep the pH stable and the upper 7's are fine for Mbuna.
My concern is your nitrites. How old is the tank? You're nearly through the cycle but I would still wait until you hit zero before making the switch. Mbuna are messy, and will add an extra bio-load to your tank right away.