Sesame Seeds are sometimes found in the same part of the store where you find Sushi.
However in most other stores you can find sesame seeds in the grocery store in the aisle where the spices are.
The sesame seeds in the grocery store I shop at are always in the same aisle that spices are so look in the spice aisle and if they are not there then look where the Sushi is or ask a store employee who can help you find the sesame seeds in their store.
Some stores stock things in different places than others but mostly sesame seeds are stocked in the same aisle as other spices are.
Sesame Seeds are usually sold for $2.59 and $5.59 depending on the store you buy them from.