Welcome to FT! As TWA884 mentioned, there are several sites like FlightAware that capture this data. However, I suspect the granularity may not be what I think you are expecting, which is to see how far your plane descended during a single, sudden drop in turbulence. If flight data is captured, say, every minute, your plane can easily drop and return to its cruising altitude within that timeframe, and the logs would not show anything.