This is one company I saw that it is selling solar hot water systems. Their web site has good information & graphs if you're looking at this, no matter what company you buy from.
It does continually surprise me how few people have installed solar hot water, when the payback is immediately net positive (not kidding!) for some of the loan scenarios. You borrow money, but you save more money in energy costs than the cost of the loan, even in the first year. Not many capital investments are this good!
Here's one type that can be easily added-on to your existing hot water system (from Sunward Solar):
Take a look at the payback graphs here:
Here's the installed cost: