Hardware stores carry a wide variety of magnetic bowls to choose from. I have 2 (two) at the front of my longarm, 1 (one) for each side, so I have handy reaching of the pins from either side.
I use longer "no head" pins to to pin down any portion of my top when quilting. Be careful when using pins with your long arm. I do not recommend sewing over pins, in any way shape or form, as sewing over pins may cause damage to your machine. If you do pin down the edges, sew to the pin, stop stitching and remove your pin and repeat. Purchase "no head" pins from your local quilt shop or department store.