Try www3 dot sears dot com and www dot discount-marine-parts dot com.
225 prefix makes it a Force-made motor. Any Merc dealer should be able to order you a manual. They were unchanged for many many years.
1989 from d-m-p
Since it's a Force, many parts will be available right here on iboats.
Owner's OB4390 from sears
Parts MB4390
I was going to make you look those up, but I'm glad I tried myself. They've expanded their offerings online. You can even see parts breakdowns. I would assume that they're printable.
Definately bookmark those sites and read-up on d-m-p. Lotsa neat info.