I'm saving this version of what I recently wrote as a text file to cut/paste, How about making this into a sticky-version we can just refer the lazy-butts to? Feel free to edit it...
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FINDING STOCK PARTS (aka: "Google is Your Friend")
To begin with, you MUST find and write down your engine's model number (if need be, google "X-brand engine model number location", where "X-brand" is the name in big letters written on your engine. ie, Predator, Honda, Tecumseh, Briggs&Stratton.
If price is no object and/or you NEED the part NOW, call your local parts store, give them the Engine Model Number and tell the nice person what part you need, then open your wallet. If you're looking for a good deal and/or can wait a couple of days, go to the next step.
Google: "X-Brand Model-Number Parts Manual" and look for an online parts manual with exploded views.
Find your part in the nice picture, noting the Reference Number pointing or nearest it.
Find the Reference Number in the parts list below the nice pictures, double-check the description, and get the actual OEM Part Number of whatever you're looking for.
Google "X-Brand Part-Number", and see what comes up. You might have to add in what the part is, like "Carburetor" or "Starter". Do some comparison shopping. Keep an eye out for ebay and amazon links, seems like most often that's where the deals are.
Note that you'll often find that the original OEM Part Numbers are superceded (replaced) with other OEM Part Numbers. This is normal - as time passes the parts suppliers look to reduce their inventory by combining similar parts/kits, and also as OEM upgrades to parts occurs. When you see this "Superceding" happen, take note of all the associated part numbers. Also, there are several aftermarket parts manufacturers like Oregon and Rotary that sell quality OEM replacement parts for far less money - and they'll have their own Part Numbers. It's a very good idea to Google all of these Part Numbers as well - you never know what deals you can uncover this way.