Is there a way to search for spells with "X" in their casting cost? I tried the cost search option but if you put in "X" it just changes it to "0". Tried Google and couldn't find anything.

Unfortunately, no, there's no filter that can search for X is the casting cost.

However, you can somewhat easily search your inventory without having to scan pages and pages of cards. On your Inventory page, go to Tools > Export, choose Extra Columns > Cost, then export. Open it in Excel (or an equivalent spreadsheet program), and turn on Filter. On the Cost column, go to Text Filters > Contains and filter for {X}. Not as fast as a native filter on the website.... but took me less than a minute to do all the steps! Takes even less if you already have a CSV of your inventory already exported (which I always recommend).

This'll do. Great suggestion! Thanks.