A mentor for what type of trading?There are plenty trading methods.From throwing darts to sector rotation.
The best way, and very hard to almost impossible, is to find a winning trader who will speak with you for free; perhaps you can work for him or her. Unfortunate Rule Of Thumb: Most "mentors" who teach for money do not win in today's markets. They often charge quite a bit up front and then want continued payments. I was in shock, many years ago, to see the bottom line trading records of the paid mentors at one prop firm: They all were losers who owed the firm money. Another firm I knew very well that was full of mentors for hire; well, the firm went under owing the traders money. And then there are the legend of vendors who call themselves mentors. Sometimes market anomalies or regulatory glitches that favor a trader (the old SOES rules) end and traders who had an edge become losers and then they become mentors. Some mentors are wild yahoo's who were lucky for awhile before going bust. Some mentors were never winning traders and are hardboiled liars and hustlers. Some are delusional idiots. I once walked into a training seminar at a retail broker where the teacher was trading on demo but pretending to be live. Sheesh. But only real rubes would fall for this one. You are smart to look for mentors. However. Buyer Beware!
Yes, working for a successful PM or a prop trader, preferably in a good institution is the best way of learning. It's hard to find and get those jobs if you are the guy who wants to learn. Oddly enough, at the same time, it's hard for the senior traders to find good juniors - there are plenty of candidates but most are either [unreliable, too arrogant, lack common sense, stupid etc]. It's the "NYC problem of dating" type of issue - girls are complaining that they can't find the right guy while guys are complaining that they can't find the right girl.
I was mentored by Ray Cahnman and a couple monster Treasury Spread traders while clearing my own account at TransMarket Group. I was mentored by Pete Franz of Lehman fame at a commercial energy trading desk. I was mentored by David Ellis - a legend in the S&P pit, while proprietary trading at his firm in Glenview, IL. So yes, if somehow you can find yourself at a firm or institution and if the financial success of your mentors is directly dependent to some extent upon your own success then yes gaining entry to a firm or institution would be ideal. I think that this type of opportunity is unfortunately much much harder to come by than it used to be. The proprietary and institutional trading business model and landscape has changed tremendously in the past decade or so. These days it seems like firms are keen on hiring coders and quants - that seems like the lion's share of posted job openings. And my sense is that writing and testing code at a firm does not automatically presume a direct career path to a trading position per se.
WIIFM is the cornerstone of all mutually symbiotic relationships - business, personal, environmental ecosystems... If WIIFM is not mutually beneficial then the relationship fails. Businesses fail for lack of customers, marriages end in divorce, and in ecosystems the biotic (living) and abiotic (non-living) states of natural equilibrium become unbalanced.
The best mentors are good trading books. Of course, this require time to read, study and finally understand.
The satisfaction of helping another human being can be it's own reward. There is an old saying..."When one teaches another, two learn."