It was hard to find for me at first too, but in the menu on top you see "shop". In a dropdownmenu there is again "shop" and if you click that button you will go to this page where you can buy some collectibles. Also in this subforum people do sell them so keep an eye on the topics.
you can consult this thread to learn about transferring TBT between yourself and other users.
as such you can use that method for transferring tbt to others in order to buy collectibles from them. there is also the shop tab, where you can buy collectibles directly from the shop with no issue. however, if you want, say, an orange or a pear, or even a birthstone, you would prob be better off buying them from someone in the TBT Marketplace because the pears/oranges sell for around 30 tbt and the birthstones generally only sell for 150-200 tbt (whereas they're marked as 299 tbt in the shop).
Haha no worries it took me also a while to find the shop on my phone Maybe it's an idea for the admins to change the button to "tbt shop" because with 2 times "shop" underneath each other it's not very clear the 2nd one is a button. In the other dropdownmenu's the top text is not a button
And forgot to answer this question but yes some are not always available like you have seen in the shop already I think. Birthstones will switch every month. I don't know how much the others will change.
Oh and other ways to earn collectibles are contests. There were 2 not so long ago where you could win a pink seashell or if you just participated in both a you got a blue seashell that's just an example for another way to get collectibles.
And then there's the bell tree fair, but I don't understand how that works. Something with earning tickets and getting exclusive collectibles. I made my account after you so I've also never participated in a bell tree fair. But this will be next week for the first time in years so I guess everything will be explained by then
Maybe it's an idea for the admins to change the button to "tbt shop" because with 2 times "shop" underneath each other it's not very clear the 2nd one is a button. In the other dropdownmenu's the top text is not a button 
