Lookin for transmission linkage 1990 buick riviera , is broken

Your best bet would be to check with local scrap yards. You'll find them in the Yellow Pages under Auto Salvage. You'll probably have better luck with the smaller ones as they are more likely to have older cars on the lot. You could also try posting an ad on your local kijiji site.
Something like transmission linkage is generic so it's likely that different GM makes and models from that year will be the same, but you'll have to look at it to be sure. Hope this helps.