Gather all the links to buy-pages of your book(s).
One way to do this is to find your book on Goodreads.com. If your book is on Amazon, it should be on Goodreads.
On your book's page on Goodreads, you'll see among the buttons two that take readers to where they can buy your book. One is named Amazon and then a second one. Amazon gets its own big button because it owns Goodreads. But it does include a bunch of other outlets that might sell your book under the other button. So click that button.
Then follow every link and see if your book is for sale there. If it is, capture that link and add it to your spreadsheet. You may want to store these links next to your book in your Assets list. Just put it in the next column to the right. Then you'll know that link goes to that book.
Move everything below that down a line and add another link to that same book.
Repeat until you've gathered all your links for that book. You can do this one book at a time or all at once, depending on your schedule.
Why go find the link every time you want to create a post? Instead, save time by storing all the links to your books' buy pages in your spread sheet. Then you'll have them all handy whenever you want them.
Hint: This will be a growing and/or constantly changing list. Another reason why it's handy to keep it in a spreadsheet where you can always add lines or columns.