How many elite athletes are there in the US?  How much do they spend
on (legal) performance-enhancing tools / substances?

I'm investigating the potential market for a (legal) product that
would enhance athletes' training effectiveness.  I'd like to know the
target market size for such a product.  The closest equivalent might
be energy gels (at the low end; how many sold a year?), or at the high
end, these new cooling devices that cool the body temperature down,
e.g., through the hand (how many do these companies hope to sell each
year?).

As I would define it, "elite athletes" includes:
>> professional athletes
>> college athletes
>> high school athletes
>> those training for a national team (e.g., Olympic team)
>> those training for extreme sports (e.g., Ironman triathlon; super-marathons)

An ideal answer would include a breakdown of the estimate of athletes,
with sources.  The best sources are "primary sources", rather than
general-purpose magazine or newspaper articles -- but reputable
general-purpose sources are better than none.

Depending on the quality and detail of information provided, there
will be a tip of up to $100.

Feel free to ask questions if you want more details before answering.