Rock Auto sells a kit of green O rings for your Reatta AC. Just look up Buick, 1990, Reatta, Heating, AC, O Rings. Cost under $10 and has much more than you need. Also found Harbor Freight has a plastic box with dividers containing many sizes of the green AC O rings.
http://www.rockauto.com/en/catalog/buick,1990,reatta,3.8l+231cid+v6,1019911,heat+&+air+conditioning,a/c+system+o-rings%2C+seals%2C+&+gasket+kits,13290?a=Referer+www.google.com+URL+%2F
http://www.harborfreight.com/205-piece-hnbr-o-ring-kit-67644.html
Enjoy. Reatta AC servicing is lots of fun.