Seems random to me - I have only done one upgrade on my Sleeping Shelter and haven't built any other forms of shelter/housing and have had five people join the community so far, at different time intervals (e.g. time between first two was a few days, time between last two was 30 seconds).

Adding to the comment by Blue Dragopn - you may need to level up a little before you find it, or wait for a change in season. I have found, after over a week of play (I'm on level 40 now), I'm finding new items when foraging. I did find garlic fairly early on in the game though - keep trying!