I'm guessing you are referring to the Restricted free agents who have not yet received a qualifying offer. That should open on June 15th.
You can see the list of available RFAs in the Free agent center. Just hit the filter button at the top left, set the filter to Restricted FA and remove Unrestricted FA.
Fair warning; these free agents aren't exactly "free" if they accept your offer. The team who currently owns their right have 7 days to match or better your offer. If they don't you'll get the player but also send draft picks as compensation to his former team. The amount and level of the picks vary based on the contract price you offered. The higher the price the larger the compensation will be.
To give you an idea of the "price" of a free agent, here's the compensation rule for the current season.
https://puckpedia.com/offer-sheet-tracker