Static length (your at rest measurement) of 15" would be the starting point to add for compression and extension. Let's use 4 inches as the extension and compression. The shock you need will be roughly 19 inches long when extended full length and 11 inches long when fully compressed. Take these measurements to your local auto parts store and ask to see their shock catalog. In the back of the catalog are charts showing shock style of mounting(stud or loop) and compressed and extended lengths. Find the closest shock to your measurements, then use the part number to application chart to identify the shock as front or rear and what car it is designed to work on. There will be many applications per number crossing manufacturer and car/truck lines. Remember you really don't wanta Chevy Vega shock on a full size truck! When you find what will work have the parts man get them for you and you're on the road!
REMEMBER THIS! Virtually all shocks have a life-time warranty from the manufacturer regardless of what brand name is on the sticker on the shock you get. Most auto parts stores carry Gabriel or Monroe. Keep tyhe part-numbered ends of the boxes stapled to your original receipt, and when it's shock time again, take the recipt and the old shocks and walk out with new ones. DOn't forget your receipt!