Here's a collection of solar chargers you can find on JC Whitney, Amozon and ebay. But it is only a small collection so you can visit these sites and select someting you think you like. I think this is an area of uncharted waters so I have no idea what size would do the best job. Keep in mind these are all 12v chargers and you have a 36v system. For the most part you can charge a 36v pack 1 12v battery at a time. However, this would still leave the other 2 batteries sulfating. You could buy 3 chargers but I not sure if you could actually connect them to all batteries at the same time without causing a big spark. Therefore, you may need to disconnect the series connection for charging 3 batteries at a time.

The unknown is still what charging rate is required to stop sulfation?

http://www.jcwhitney.com/solar-power...wx?filterid=j1

http://www.amazon.com/Wagan-2017-Sol...930004&sr=8-19

http://www.amazon.com/Sunforce-50022...930161&sr=8-21

http://www.amazon.com/Coleman-50005-...930161&sr=8-30

http://cgi.ebay.com/Solar-Battery-Ch...item45ee2dd7ed

http://cgi.ebay.com/12-Volt-SOLAR-AU...item43a230fbcf