Jamie,
Went there in October. Absolutely fantastic place. Don't know how long you are there for but you have to run in Central Park. Its an experience i will never forget. If you have time, do the 'Big' loop (6 miles, very hilly) as well as a few laps of the reservoir.
anyway...thats not your question.
I bought some stuff in Niketown on E 57st (just off 5th Avenue). It was huge - don't know how it compares to Regent Street.
The other place I went to was 'Super Runner Shop'. They are scattered all over Manhattan, but I weny to the Grand Central Station one.
Foot Locker appeared to be the other main chain retailer, but I ddidn't go to one.
Don't know if the stuff is different to what you can buy in the UK, but it felt American anyway!
Have a great time.
Mike.