What is a good lightweight (gel cell?) battery for a Cessna 170B?
Where is the best place to get it?

Contact BCP member 182 STOL. He offers the Odyssey style long-life battery as well as any needed hardware to install it. If you have a rear-mounted battery, he offers a higher quality, longer lasting firewall mounted battery box. If you already have a firewall mount box, he has the battery and can help with any field approval or "minor alteration" paperwork. This type of battery offers several very important advantages compared to the liquid type batteries or the old "gel cell" types and is just about universally regarded as a worthwhile upgrade.