Dealing with 15-year-old production lines that manufacturers have abandoned can be really frustrating! I've been there - it's like trying to keep a classic car running when the parts are no longer made. Based on what I've learned, here's a practical approach that works well:

First, do a criticality analysis - identify which PLC parts are truly mission-critical. Focus on components that would cause complete line shutdowns if they failed. These are your top priority for stocking.

Second, build relationships with specialized suppliers who deal in legacy parts. Companies like Industrial Automation Co. specialize in verified inventory and cross-brand replacements for obsolete equipment. They can help you find compatible parts even when the original manufacturer has stopped support.

Third, consider a hybrid approach: keep critical spares on-site for immediate needs, but also identify reliable suppliers who can get you parts within a reasonable timeframe. For less critical items, you might use a just-in-time approach with trusted vendors.

Fourth, document everything! Create a cross-reference guide showing which alternative parts can replace your obsolete components. This is especially helpful for things like I/O modules where different brands might be compatible.

Finally, think about long-term planning. If certain parts are becoming impossible to find, you might need to budget for gradual equipment upgrades or consider retrofitting with newer, supported components over time.

The key is being proactive rather than reactive - it's much easier to plan for these challenges than to scramble when something fails unexpectedly!