I used to live and dive the area a lot.
Tarbert is the best slip, use between ferries and park the the car park beside it, no problems.
Further up is Minard which a a beach launch but no problems.
Slightly further up is a fish farm that used to allow launching at weekends without much problem if you asked.
Even further up is the caravan park but this costs (all others are free)
On the other side is Strachur which is a beach launch again but easy enough.
You should be able to use one of these, don't know ardrishaig but our club always went for free before paying for a launch, plus there is half of bugger all around Lochgilphead, its a dump!