Scripts/method for tracking where a client has came to my website...?

Hi pros! The objective is as follows: A client that has already bought a product from my website needs to be tracked where initially has came to the website ( did he clicked on FB advertising, Google ad, Banner ad, Google organic search, Referral, direct, etc... )

Is it going to work if I copy/paste some sort of script on my "Thank you page" to which he is led after he did the payment? And also on a different page in a chart for me to be shown where exactly has each one of the clients that has been on the "Thank you page" came from. (of course there will only be people that has already bought and paid and redirected to the download/thank you page)

I do have Google Analytics installed but there only shows me the number of visitors in the whole website , how much and where did they came from. Is there an option for this more detailed info on Google Analytics ?

I'm not selling via e-commers system I have paypal buttons.

Initially I'm looking for a simple free solution since I still don't have much traffic and when I develop I'll advance into more detailed and paid versions of tracing like Clicky Spy.

Thanks: Ivo