Try your local Electrical wholesalers. Ask for "Piro cable" clips they look like an open letter "P" They come in sizes for 1/8" or 3mm to over 1/2" or 15mm plain copper or soft plastic coated usually Red. and are plenty strong enough to do the job.
Added note. Not wishing to knock Twisterowner's good response but the piro ones are much stronger then pluming ones.