Is there a place where you can buy a plastic cap for a bottle?

I don’t know about the specific bottles you are talking about, but there are definitely stock sizes for caps. I re-use bottles for all sorts of things, and have found a lot of tops to be interchangeable.
Maybe check Amazon.

Amazon has a 3 cap package. The brand name is VersoCap. It’s 17.95 . To be used on shampoo, lotion and hand soap to get every drop out.

Thanks, CandidMajor and Chyna. I placed an order. The price is somewhat outrageous, but in the long run the convenience makes it worthwhile. I will have to be careful not to lose the caps again.

Or you can buy bottles of products for the caps and get the product included for free in the purchase.

The problem is that the product caps are screwed on. I needed something that could just be squeezed over the container, which is what I got from Amazon.