If you buy a new Saveplace® furniture hammock with 4 straps, you will find rubber bands in the package but if someday you need the new ones, you can buy them here.
Saveplace® hammocks are designed to be attached at your desired height because of the adjustable fastening straps. Pay attention that all straps are at the same level.
You do not need any furniture crossbars for the hammock to hold. Except in case of very slippery or very narrowing furniture legs, use rubber bands: first put bands on the furniture legs at the same height, then attach hammock straps on them).