I have been using my Solara pool heating panels for more than 3 years and I am very happy with it. Just ensure to get a solar controller with it.
Solara Pool Heating System Reviews
It is typically sold anywhere dolphin cleaners a re sold (including Amazon). It can also be bought as a DIY installer. The panels are roughly 2’X4’, cost about $300 a panel.
ALSO SEE: Which is better, kreepy krauly vs Barracuda? FIND OUT
The good thing about it is that you can connect 2 or more panels together and they have a plastic cover that goes over them so it is glazed (protects against freezing).
They work really well and they warranty the tubes on their panels if they freeze. The pricing is affordable but not cheap too and installation is easy! They are reliable and have a great customer support.
Other alternatives will be Heliocol and Techno Solis. The Heliocol system panels provide some of the lowest life cost of any solar pool heating system other than their commercial system.