There are no plants to "absolutely" avoid, save perhaps, Duckweed. Once in your tank, it'll take a long time to get rid of, as if you leave even one plant, it will grow.
I get my plants locally, mainly. Another good method is to look through the Swap and Shop forums for deals.
As for quarantining plants, it depends on what you are trying to accomplish. If you are worried about fish parasites/diseases, you can keep the plants in a plant only tank for about 3-4 weeks to ensure no parasites are present.
If you are worried about algae/snails, there are various disinfecting methods (such as a diluted bleach solution, potassium permanganate solution, etc)
Firstly, welcome to the forum! Your first question really depends on a lot of different factors. Do you already have something set up or are you in the planning stages? The answer can depend greatly on what type of lighting you choose (low, medium or high), whether or not you choose to inject CO2 and your water parameters (PH, etc).
As far as where to get plants, I, like most others here, buy them from the Swap & Shop here on the forum. Lots of people post packages or individual plants for sale at great prices and they are generally higher quality than anything you would pay for from a commercial seller.
Quarantine of new plants is not necessary but a lot of people like to use a bleach dip to get rid of algae when they first receive them. I personally don't do this and just add them straight to the tank unless I see unwanted algae on a plant.