Yes, you can buy a stripped lower receiver in California. However, there are some important things to keep in mind before doing so.
First, it is important to know that a stripped lower receiver is considered a firearm by the ATF. This means that it falls under all of the same regulations as a complete firearm. That includes the requirement that purchasers be 21 years of age or older, and that they undergo a background check.
Second, while there are many reputable dealers of stripped lower receivers in California, it is important to make sure that you purchase from one that is reputable and has a good track record. There are some unscrupulous dealers out there who sell inferior products or even (in rare cases) stolen goods.
Third, it is always a good idea to consult with an experienced gunsmith or other firearms expert before buying a stripped lower receiver. They can help you make sure that you are getting a quality product and answer any questions you may have about the process of completing the build.
Fourth, when purchasing a stripped lower receiver in California (or anywhere else), be sure to follow all of your state and local laws. This includes ensuring that you are carrying the proper identification and paperwork with you and that you are not violating any other laws (such as those regarding firearms in public places).
With these things in mind, buying a stripped lower receiver in California is a perfectly legal process. Just be sure to do your research and purchase from a reputable dealer.