Sand particles are the main building blocks of cob. Ideally, each particle will have rough angled sides so the sand pieces interlock with each other, helping to hold the cob together. If the sand particles are rounded and smooth like ocean beach sand, they'll be more likely to slide off each other.
Sand that's made up of various sized particles makes a strong cob mix. The pieces can vary a lot, I think some pebbles and small rocks also strengthen the cob. Don't be too fussy about this. If in doubt, make test bricks to see if what you have will work.
If the soil from the site is short on sand, have a look around your property. Soil can vary a lot even within just a few feet. Often the movement of water in streams or rivers, past or present, has left sand deposits somewhere.
If you need to buy sand, go to a concrete supply place and ask for coarse concrete sand or 'reject' sand. In most places it's pretty cheap. For an extra fee they will usually deliver it to your site in a big dump truck, if you have a decent access road. Or you can haul it yourself, a truckload at a time. The gravel yard will load it for you. Don't let them overload your truck! If you can't drive all the way to the cob site, take it as far as you can, then load it directly into your wheelbarrow or garden cart to carry it the rest of the way.
It can be tricky to figure out how much sand you'll need. It depends on the size of your house, the thickness of the walls, the number and size of the window and door openings, and how much sand you'll have to add to your soil to get a good mix.
Remember: if you plan to make an earthen floor, sand will also be a major ingredient of your floor. You can roughly estimate all this by volume. If you're getting the sand yourself as you need it, it will be less important to know the total amount. If you're having sand delivered, ask what the minimum delivery is. If that seems reasonable for your plans, order that amount. You can always get more later if you need it. If there's extra, you can probably use it for landscaping, gardening or future building projects.