Not so much amp sims as there are cab and rig sims. The HeadRush Community Forum offers free downloadable presets. It's likely that Fractal, Helix, Kemper and other modelers offer free presets from their community forum websites as well.
Caveat Emptor: Neither the forum nor HeadRush assumes responsibility for whatever you download. All presets are intended to be uploaded in good faith, however, neither the forum nor community assumes responsibility for downloaded malware or viruses.
Most reliable cabinet IR downloads are paid-for downloads. Some more generous with their mic positioning and speaker variety than others.
Doc McFarland from his YT channel offers a wide range of IR downloads from the Sonic DNA website for various devices, and at a reasonable price. Other websites (Ownhammer, RedWirez, Choptones, York, 3Sigma) offer better quality IRs with less variety, with a slightly higher pricetag.