Can stored/leased 737s be used to substitute for grounded MAXs? The 737 MAX was introduced relatively recently. I would have assumed they were replacing at least some older 737 models in operation with the major airlines that took them (eg Southwest, American, United, in the US). Did those airlines immediately sell off their older planes as the new ones joined their fleet? I see reports of huge flight cancellations from lack of the new aircraft. Since they have common type ratings with the earlier 737, what happened to the older planes that none of them can be pulled from storage or dry leased and put back into service? Surely this grounding is long enough to be worth the transient expense?
It really isn't 1 new aircraft = 1 mothballed aircraft. Most of the 737 MAX aircraft were fleet expansions (for US carriers). The older aircraft get retired when they aren't economical to fly anymore, and even then a retired aircraft is often sold either to a scrap yard or to a lower budget operator. In order to bring an aircraft back into service they may have to go through C or D checks before they can be considered airworthy again (because old aircraft are often retired at a D-Check when it needs a major check). 

Airlines need to schedule C/D checks out months in advance, each aircraft costing millions to go through the check and a month or more of downtime. There is only limited space available to do these checks, so even if they wanted to bring 10 aircraft back into service, it may take 6 months and cost $20-40 million. It's cheaper to cancel the flights.