Search for "X spells"

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).