Can the donor hair be taken from the sides of the head. I don’t want to thin the back of the head any further.

It is best to ask your surgeon about operative issues as each doctor has their unique regimen.  Your doctor also needs to examine you to make sure what you are asking is possible.

When treating hair loss with FUE and FUT techniques hair can be harvested from both the back and the sides of the scalp. 

The donor hair can be harvested from the horseshoe which consists of the occipital and parietal scalp (back and sides). There is a limit to harvest of approximately 10-15% of the available follicles in order to avoid thinning at the donor site.As always, discuss your concerns with a board certified plastic surgeon (ABPS) with experience in hair restoration.

The short answer is yes, we normally remove about half of the grafts in an FUE procedure from the side donor hair.

The area from the sides of the scalp is often a valuable area for harvesting grafts.  And it may allow your scalp to be more uniform.
Dr John E Frank

You absolutely can use follicles from the sides of the scalp. When we refer to the donor area that includes the back of the scalp and the sides of it as well. Generally we prefer to use the back of the scalp because that area has a higher density and better quality of follicles. Obviously we adjust accordingly to each patient.

Yes, it's common to also harvest above the ears. Otherwise, the back side of the head will eventually be overharvested. 

Yes, the donor area can be taken from the sides of the head both in a strip surgery and in an FUE surgery