If you want to play mad scientist, here is a list of how the BJCP recommends on doctoring beers to taste off flavors
guideline for doctoring beer
If at all possible, I HIGHLY recommend finding some sort of brew club. The club I joined in Cleveland had several National and Grand Master level judges. When I did the BJCP study group with them, it was like having a beer tasting Sensei. For me at least, I had to be told "that is oxidation" etc.
You can also order a kit that is a little more encompassing than the beer voodoo in the BJCP study guide. I think its through Seibel and costs around $120 for AHA members. If you organize a BJCP study group, prove you have a group meeting every week and have an exam scheduled, the BJCP will send you one for free.
No matter which way you go, you should only doctor light american lagers. We used Bud Light because thats what is available. I think they use Bud at Seibel. Miller High Life is a good one too. Basically, it should be as bland and flavorless as possible to let the off flavor "shine through".
Another thing you can do is buy commercial beers with known off flavors.
acetaldehyde - budweiser
oxidation - Anchor Steam (if you are on the East Coast), most imported Belgian beers, especially Duvel (unfortunately).
diacetyl - Red Hook beers (or so I'm told - I can't taste or smell diacetyl)
sour - lambics/flanders red (not an off flavor in these beers, but it will teach you what the flavor is)
I'm sure there are others can chime in with commercial beers that exhibit certain off flavors too.