On a budget
Magic pie 4 Hub Motor Kit Magic Pie V4 Complete Kit E Bike Conversion EBike Kit 1500W 30MPH | eBay
the optional battery seems a little small in Amp Hours. The controller is inside the wheel which is nifty.
,
If you want regular battery that'll bring it to around 26mph-28mph\
then Calibike.com 48v 15AH at $450.00 inclusive is good.I just picked up one and it's
working fine.

For more performance
Luna Cycles 52v 12mah will give you the 30mph on the Magic Pie
but it's $150.00 more expensive and may not give you the range.

There is a Chinese Manufacturer Ping Battery and his product is very high quality but he's pricy and you'd have to import from China. I had the battery and was very satisfied. He uses Lipo4 packs instead of Headway a 123 or 18650.

You will need to mount your battery on the rear rack because a triangle center mounted battery won't fit on
a dual suspension mountain bike.

Good Luck!