craigslist - it cost me 20 bucks for 1 and 100 bucks for the other - both have worked great. the 1st one was from a force 85 motor i bought for 200 bucks - took the lower unit off and sold the engine for 180. The other one I got with a slightly bent skeg - it was easy to straiten and it on my boat right now b/c it matched better (white)
if you are not willing to chance it there are some on ebay that are from people who check them out and sell them with some kind of return policy.
a 20" from a 80's 85 or 125 horse should fit too. things to note. 2 piece are 1.93 ratio, 125 hp (stamped 4) are 1.73 ratio and 85 hp (stamped 3) are 2.00 ratio. Some are single exhaust like mine in picture below. Just change the impeller and gear oil before you swap them.
pic of 100.00 unit on my boat:
