A lot depends on how / where you are going to use the boat, and how far out to sea you are going to go.
I took the view (rightly or wrongly) when I bought my SIB and outboard that reliability was important to me, so ended up buying a new 4 stroke Suzuki 9.9HP from Pacer Marine:
At the time they had a batch they were selling with a big discount. They also stock some used outboards.
I started out deflating / inflating the boat and carrying it in the back of the car (Ford Focus) with the outboard. The weight of the outboard was a bit of a hassle, but to be honest the fact that it made the whole car stink of petrol was more of an issue - that and the fact the boat took up the whole of the boot and back seats was more a factor in changing to a trailer.
I get my outboard serviced at Fairweather Marine in Fareham - he usually has a selection of second hand outboards:
Home Marine in Emsworth also has a good reputation (no website):
Ron Hale Marine in Portsmouth has a selection of new and used outboards (I have heard mixed opinions of them):
There is also BHG Marine:
Further afield, Seamark Nunn have a selection of used outboards:
Bill Higham Marine has a wide range of outboards - I have no direct experience of them, but reports I have heard have not been very flattering - prices may be a bit high: