I am looking for a 2018 Lidar DSM (not DEM) for Texas, US. Do you know any source from which I can obtain the data?
Try OpenTopography.org, they index open LiDAR datasets from around the world (as well as hosting some of their own).
Check out their data map to see extent.
They show a lot of US national LiDAR datasets over Texas. These are available through the 3DEP program which is hosted on AWS S3. Important here is that AWS Public datasets are often "requester pays" for data transfer costs.
Also, many may have the original LiDAR data instead of processed DSMs. If that's the case you can always generate your own DSMs using PDAL in Python.
The State of Texas has what it is called Texas Natural Resources Information Systems. Their site is TNRIS DataHub. Here it is https://data.tnris.org/
They might be lidar that are smaller area if you are seeking smaller areas .