Definitely creative searching! Here is my method and how to find it.
I didn't like the free version because the formatting did not indent paragraphs and with the short dialogue the text felt all crammed together.
You can find it by a search on the Amazon website with targeted keywords. In the Amazon search field, select "Kindle Store" from the drop-down choices and type "of human bondage modern library" in the search box and it will bring up the book. The ASIN is B000FC1JOY. ASIN is Amazon's unique identification number for the book. You can just paste that into the search field too.
When there are too many versions of something then I start typing publisher names like penguin and oxford into the keyword searches. Sometimes I go to the paperback version and then look for a comparable Kindle edition from that page.
Here is another technique that works well. I download a sample of one of the book versions to my e-ink Kindle. Then from my Kindle, I select the option to view that book's description in the store. On the store page under the book image, you will see a hyperlink that says "All Editions" and a number. Click on the hyperlink and then you see a list of other options which is usually much higher quality than the one you get from the Amazon website originally.
So the way I found this particular version originally was that I located a paper version of the Penguin edition on the Amazon website from my laptop. That led me to ebooks from Penguin and Signet, and I downloaded samples to my Kindle Oasis. From viewing one of those samples on my Oasis and looking at All Editions in the store, I was able to find the Modern Library (Random House) version.
They sure don't make it easy! I hope that makes sense.
EDIT: If you still can't find it, just PM me.