Real Estate Agents who specialize in land

My husband and I are frustrated with the housing market.  We continue to get out bid on less than desirable homes that we can hardly afford.  We are starting to shift our attention to buy and build.  Specifically we are looking into prefab homes.  Unfortunately, our realtor is not supportive nor knowledgable in selling land lots.  Can someone please recommend a realtor who specializes in selling land?  We are open to anywhere in the Bay Area but are mainly focusing our search in Castro Valley area.

Thanks in advance.