On the top menu click Shop. There will be a left side menu so you press Inventory. There should be a Gift option. If the recipient wants a message, you enter what they want so it shows up.
As for restocks, it varies depending on the rarity of the collectible. Some get restocked every few months, some once a year, and others are for a limited time (1 month with cosmos & pansies for example), and some will be available for a long time. Some get announcements and others are random. Be aware the rarer collectibles can have very limited stock so I recommend raising up TBT in case.