Buy a cheap one (EverStart is the name I think, I am not 100% sure) for $25 from WallMart, they work nicely and are very cheap.
You need to buy a battery tender though, the battery charge drains fairly quickly if you let the bike sit for several days. I bought mine from (surprise!!) WallMart for $20, it is a 1.5 A battery tender, hooked up the battery with the connector sticking from underneath the seat, any time the bike sits for a few days, I connect the battery tender for an hour or two, it has an LED indicator, when it turns green I know I am done, the battery tender keeps it in top shape.
Yuasa are fine batteries, but a little pricey.
Your call now.