How do I get just the marker stakes?

There are several ways to obtain all expansion materials. You can find them in the red toolboxes which can be found on every farm, and you don't have to open them with Diamonds, some of them will open by themselves. You can trade other items for the ones you need, with your friends or members of your neighbourhood. You can buy them in the newspaper/other player's stalls, if you're lucky and quick enough to find them there. You can win them on the Wheel of Fortune. You can harvest them by growing lots of wheat. You will see that as you grow lots of wheat and harvest it, you'll harvest valuable materials occasionally too. You can also find them in the mystery fishing nets, those are the purple ones which cost five diamonds each to produce. You can also get them as rewards from your pets, and now also from the visitors to your town.

It's worth it to bear in mind that most people's farms give them more of a certain one or two items, and make it hard for them to find a certain one or two items too. For example; my farm gives me mostly bolts and nails and deeds. I struggle to find planks and screws. Marker stakes are usually the easiest land expansion material for me to find in the newspaper and people's shops, but is expect with the recent update that people will be hoarding all of their materials to use when expanding their new towns.