I find that seaweed found in Half Moon Bay and Cresent City tends to be a lot cleaner and less contaminated but you can go to the San Diego harbour, Ocean Beach, Imperial Beach, Sunset Cliffs, etc. and get you some. I occasionally grab me a good handful of seaweed to take back home when I visit Diego to crab and fish. Not sure about juicing the raw product and foliar spraying with it. Never tried it that way but it's not that easy. How I use the raw seaweed is mix it (along with fish and crab shells) into my soil.
Keep in mind, seaweed varies from one species to the next and are not created equal. Certain species are better than others. Also, the different content of active material and nutrient in the end product will be obtained by the different methods - i.e. chemical hydrolysis, physical extraction and biological fermentation. Unlike building soil and diy nutrients, pesticides, etc. extracting kelp and other raw sea material is a complicated process and can get pretty involved. I'm not a scientist and don't intend to be one so I stick with the bottled stuff. It's just much easier and i get the actual active ingredient that i want from it.
I'm currently using GrowMore Seaweed Extract (California made) and Nectars For The Gods Bloom Chaos (Oregon made). I alternate between the 2. Nitrozyme is another really good one but is a little too expensive for me. GM's Seaweed Extract is cheap as shit and works as good or better than most expensive brands out there. And it can be had for around $20 a gallon, which will last you a pretty long time. Bloom Chaos is the shit but is more on the expensive side. It makes your lateral branches and budsites pop like crazy. If you add humic acid, namely Bioag Ful-Power, to the mix, you've just created rocket fuel for your plants.
On a side note, when foliar spraying with kelp, there's so much more benefits other than keeping the plants squat. You greatly reduce plant stress, reduce shock when transplanting or taking cuts, increase nutrient uptake, increase growth rate, encourages root growth and root mass, increases flower production, increases aroma and flavor, increase overall quality of health, and I'm just naming a few.