Portola Valley Hardware sells inner tubes and a few bike parts like brake cables.

Also now's the time of year to watch out for star thistles which can puncture tires. So after a ride check your tires for glass and other sharp materials.

Star thistle spines are too weak to cause punctures, but goathead thorns can and do.

By and large, in my experience bikers tend to be a caring bunch. Whenever I've found myself in a similar situation there have always been people who've offered to lend a hand or a spare innertube to get me back up and rolling. I usually carry tools and spares with me so that I can help myself or others in a similar situation so I've not had to rely on help but it's nice to hear someone publicly express gratitude for such assistance. Just pay if forward when you get a chance (and, if you keep riding, you will get a chance).