Matt,
As above, there are cheap ones from the $2 shop and usually they are not reliable but are cheap, great for simple stuff like in car wiring etc. Then there are the UV rated (and many are not no matter where you get them from), quality zip clips (usually from electrical supply outlets) which work well. Some have stainless clip in them and some have plastic, however if they are quality they all work well. If you are not sure, zip one up a bit and then try very hard to open it by pulling on it. The cheap ones run back open easily, the good ones don't.
As above, there are cheap ones from the $2 shop and usually they are not reliable but are cheap, great for simple stuff like in car wiring etc. Then there are the UV rated (and many are not no matter where you get them from), quality zip clips (usually from electrical supply outlets) which work well. Some have stainless clip in them and some have plastic, however if they are quality they all work well. If you are not sure, zip one up a bit and then try very hard to open it by pulling on it. The cheap ones run back open easily, the good ones don't.