Technically, all your grid has to do is last long enough to plant for spacing purposes. You can remove it after that.

I've bought wooden shims from the hardware store and used them to mark ends of rows in a large garden. They would work well to hold your string or twine to mark the grids, plus they're cheap. And like ribbit said, you can remove the whole gridwork after you've planted, or leave it in place for the season.

Sandy

I've even used a tape measurer to plant before after my twine rotted. I was in a hurry and the garage is right there.

A buddy of mine use bamboo posts to mark lines in the dirt where she's planting and then the lines wash away.

There are plenty of trmporary options. Good luck and great work getting into a community garden!

I didn't use any sort of grid. Granted I have much more space than likely you're talking about, but I simply said "Ok, I need a path...riiiggghhht...HERE." And started digging. I was fortunate that I apparently have a decent eye for digging a straight line.

This is the result (obviously this can be scaled down to fit your needs):

Eyeball it, the plants will never know!

You could try plastic tent stakes, you can find them anywhere that sells tents or camping supplies. They are inexpensive and hold in the ground very well.

Good gardening, Mary