What is the best sealer to use on furniture that has been painted with chalk paint?
You have two options, wax, which has to be reapplied every so often, or an Acrylic waterproof sealer. If you want to use polyurethane, you can but be aware that it yellows.
You can buy valspar sealer in a non-yellowing formula
Do you know the best place to get it?
I love water-based polyurethane in matte...keeps the matte chalk paint look. I don't like wax because you have to redo it every so often and I'm lazy :)
Oil based poly tends to yellow while water based doesn't.