There are many to choose from. You can look at this site for reviews:
http://www.webbikeworld.com/motorcycle-battery/I would recommend a "smart" charger: one with multiple modes, to slowly ramp up, top off, then actively monitor, and switch modes, as needed. The unit I bought, for three bikes, was the
Battery Tender Jr. The
Battery Tender Jr. comes with a permanent hook-up for the battery, allowing you to quick connect with a plug, after each ride, and for storage.
As for price, the
Battery Tender Jr. can be found, on sale nearly always, somewhere, for around $28. There are less expensive models out there. Just be sure to find a unit with several modes, one which is microprocessor controlled. They only work if used religiously, though, so if you do not plan to always hook up after each ride, you will not gain their full benefits. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII