Hi Andrea. You can try www.craigslist.org, then go to California, San Diego. Go under Real Estate for Sale, then type in Alpine in the search box. I came up with 66 properties (homes & land).
Also, you may just want to drive around the area and find FSBO yourself. Once you find one, hire a REALTOR to represent you in negotiations with the FSBO. A REALTOR will be able to advise you on comparable sales so that you pay a fair market price for the property.
You may also want to check the local newspaper. Better yet, contact an experienced REALTOR and tell them what you are looking for. Just because it's not listed in the MLS doesn't mean it's not for sale. Being a full time REALTOR, I know of properties that are coming up for sale but aren't listed yet and properties where the owners are getting divorced and will need to sell, as well as properties that the owners have said, "if someone gives me x amount, I'll sell."
I hope that helps.