The standoffs are included in the box o' goodies that comes with the case. There are two sizes of standoffs that you might use for the vertical GPU mount; which one you use will depend on where you mount the GPU.
For the outer mount, closest to the glass, you'll use the smaller standoffs. These are the same as the motherboard standoffs and you have 3 of them in the baggie. These will be about 9.4mm in height (total) with a 6.5mm standoff from the base.
For the two inner mounts, you'll use the taller standoffs. There are only two of these. They are 12mm tall (total) with a 9mm lift from the base. They are noticeably larger than the other mounts.