It's actually better to have it made and installed by the same person. That way if the measurements are out or something goes wrong, you are not liable to pay the installer for his time over and above his quote because he couldn't do the job first time round.
In fact, even though it seems cheaper to buy your own product for any part of your reno (e.g lighting, appliances, flooring, tiles etc etc) it is often cheaper and better to use your tradies to purchase. They may put a margin on, but they will also often get a trade discount so your retail price is comparable to their trade discount + margin. Does that make sense?
Also, if there are ever any problems with products supplied by the installer, they will normally take responsibility of replacing the faulty item and installing it within their quote (assuming you have a fixed quote which you always should!).
Sorry I have rambled a bit but hopefully that gives you a bit of useful info!