I always get mine done professionally - the difference isn't enough to make it worth risking messing it up (and if my Aud's mould doesn't fit correctly they redo it for me for free)
One tip, when you go to your Audie to get new moulds made ask them if they charge for the impressions (I found most don't).
If they do charge more than a small fee, tell them that you want the impression mould returned with your new HA moulds ... that way when you need a new ear mould (from wear & tear/age) then you can send that impression in and have them just remake the new HA moulds off of those same impressions.
Honestly, for the extra service (free tubing, check ups etc) and help I get from my Audies, I'd rather continue to get my moulds done directly through them instead of trying to save a few bucks a year on "make your own moulds". The other concern I'd have would be the quality and warranty (ie if they break/split etc) of going through a discount place.
For me, having very comfortable (& properly made, including a custom length it goes into my ear canal) is very very important and makes a huge difference on how much I wear my HA and how well I'm able to hear/understand what I hear.
If you decide to try it yourself, good luck, however honestly I wouldn't do it.