sadly dimensions threads are kit only and not available for sale ...if its for a full pattern youve downloaded its best to use dmc/anchor conversion charts..they are a pretty good match
if you are using a bought kit tho and have run out of thread you can contact dimensions and they will send extra thread free of charge so you can finish
You can usually email or call Dimensions to get more thread, but that is limited. Otherwise, there are charts for conversion to Anchor and DMC available. Id take the thread to the store and match it against threads there to get as close as you can then stitch in a different area.
Dimensions kits use DMC threads so if you run out or require more use a conversion chart. You use to be able to call a telephone number on the kit but now Dimensions just tells you to find a conversion chart online