I looked at a few of Amazon's offerings, as long as you're talking about the small battery banks for charging your phone, etc. They had them ranging from $30-over $50.
Most of them don't recommend solar as the primary charging method, primarily due to how long it could take. I think it is a good idea to give you the option.
I have an Anker 8,000 maH that I carry when I hike, camp, or whenever I'll be away from the house/car. It is small, light, and charges my Samsung S6 several times.
Goal Zero also... a little pricey but they seem to work well.
I've had a small Nomad 7 kit since 2013 or so and it works like a champ every time I test it. Not very large, but it'll charge 4 AA batteries or a cell in a couple of hours of direct sun.