That's weird because I just went on to Google and searched 2018 Buick LaCrosse drive belt and they pop up over the place, gmparts.com Amazon, ebay, Walmart, AutoZone, partsgeek, stockwiseauto, finditparts, truckid, and like so many more websites. I always buy my own parts and then bring them to someone if I don't plan on installing them myself. And the belt for your lacrosse looks a lot cheaper than the belt was for my 2011 lacrosse. You should be able to just go to GM parts search the part number for that belt and then get on to Google with your make model year that part number and you should have a whole bunch of options. Or you could just call a GM dealership ask for parts get the part number for that belt and search it online that way as well. My usual go to online auto parts store is A1 Auto.
I've always found the best prices and really good quality parts from them I've never had an issue going through them for parts. Most of the parts I bought from them are brakes, pads, rotors, starter, alternator, and other small things.
Here's the link to GM website with your 2018 lacrosse drive belt pulled up gives part numbers so maybe start there.
www.gmpartsonline.net
I guess that's why this seems a little confusing to me, it just doesn't make sense that a shop can't locate a drive belt for a Buick that's 5 years old. It's not like they're getting parts from Germany or from some special manufacturer. Well hopefully some of this info will help you out.