Where can I find the contracts of my old transactions?

You can find older transactions on the transactions screen. On this screen you'll see an overview of all the portfolios you have transactions on, and also of all you entities like holdings if you also own those.

On any of the portfolio you can click on the transaction history to see all older transactions you made.

See the image below for an example

After that you'll need to click on "See overview" and after that click on "Contract".

This is another way to locate the transaction contracts.