1977 means MC-1. As you're also replacing the shift cable you have a few options. As far as the best quality, I only used non-OEM once, many years ago... Since then, never anything but....
So, your options.
1. Just the bellows you want (uni joint) part number is -60932A4 (only comes with the hose clamps). I would buy a shift cable with a bellows and bell-housing gasket if you don't need/want to change the exhaust bellows too (part number -865436A03). Total about $155.
2. Buy a kit with all 3 bellows (uni, exhaust and shift), a gimbal bearing and bell-housing gasket, part number 30-803097T1 (called a 'Transom seal kit'). For this you just need a bear shift cable kit (because the shift cable bellows and the required gasket is in the transom seal kit), part number -865436A02. Total about $275.
And as I said, I would not consider anything but OEM with any of these items.
You're also going to need bellows adhesive...
Buy from any Merc dealer or on-line store. I would avoid ebay and the like as although they use the OEM numbers, the parts are generally not OEM...
Chris.....