Based on this product's specifications, here are some alternatives and similar models
Battery pack for HP EliteBook 840 G5 — 11.55V, 4113mAh
Summary description:
Replacement battery pack compatible with HP EliteBook 840 G5. Rated at 11.55V and 4113mAh, it provides portable power for mobile work, presentations and travel. Designed to fit the laptop model listed, it restores runtime for typical office tasks, web browsing and media playback. Easy to install and intended as a direct swap for the original battery.