Can anyone point me in the direction to where I can get some dirty knickers 

I recall some profiles on adultwork and escort rankings where girls were offering that sort of transaction. The best option is to search on adultwork and on here, if i bump on one i'll let you know.

On a date a lady has perhaps worn her knickers for a few hours and anointed them with her juices and scents so we get a little damp patch so full of that musky scent that arouses us.

Escorts knickers often smell strongly of fabric conditioner, that doesn’t appeal in fact it’s a turn off.  

I suppose it the nature of the service, they aren’t aroused and there working knickers see so much action!

Of course it's the nature of the service, imagine you see 5 ladies in one day, would you be aroused with all of them? With the first two, probably, but with every next escort you'd have a problem to get your peer hard.

@OP

Just google 'knickery', and the website with that name should appear at the top of the list. There you can find a lot of such accessories. From morning pee to used dildos.