Re: 1957 Sea king 25 hp, parts
Your motor was made by Gale Products Division of OMC. Two other divisions of OMC were Johnson and Evinrude. 90% of the parts used on your motor were also used on Johnsons and Evinrudes at one time or another.
Laings is a good company. So is our host, iboats. iboats sells the same (aftermarket) stuff as Laings, with more options too. For stuff that iboats doesn't have, try an Evinrude dealer.
The best favor you can do is get yourself a parts book for your motor. Armed with the original part numbers, parts are fairly easy to find, if not totally obsolete.