Welcome aboard!

I suggest a visit to a local heavy duty truck service or dealership parts dept, not necessarily specific to GM. The part you need is in a package used by several manufacturers. I did a web search for heavy duty truck parts and found many references to the part, but no USA sales outlets that didn't require an account to search inventories (probably where you'll end up, if online). They all listed phone numbers. I found a Sachs brand (3182000009) online in Europe, for $1200 USD. USA references suggest the part is priced $150-200, if you find one in stock.