PWM lithium-ion charger
The cheapest way it to get a CC/CV Power Module from ebay.

For less than $2 you could get a good one which looks like this:


For less than $6 you could get a fancier one which looks like this and which allows to monitor the charge better:


If I needed to charge many cells I'd get 10-20 of the cheaper modules and create a rig with those. They have decent LED indicators for how charge is going.