Online sheet music shops:
http://www.musicroom.com/
http://www.printedmusic.net/

Good shops for instruments and accessories:
http://www.dawkes.co.uk/
http://www.prozonemusic.com/
http://www.johnpacker.co.uk/

Click here for advice on instrument care.
There are loads of youtube videos on how to clean your valves and the inside of your trumpet - have a look, or feel free to email me for advice. Here are a couple of videos:

 

Essential Accessories:
Valve oil:
 http://www.dawkes.co.uk/accessory_search.php?category=trumpet&header=valve%20oil&query=valve%20oil
(I use Al Cass; Denis Wick Advanced Formula is also good; anything else will do the job, too!)
Brushes: http://www.dawkes.co.uk/accessories-trumpet-brushes.html
(Recommended: flexi brush and budget mouthpiece brush)
Trombone water spray and slide cream:
http://www.dawkes.co.uk/accessories-trombone-lubrication.html 
(superslick slide cream and spray bottle recommended - or Slide-o-Mix 2 Part Slide Treatment for more advanced players)
For FREE slide grease (for my pupils), give me a small pot - old-fashioned camera-film container is ideal - and I'll give you some in there.

BRASS REPAIRERS
Will Spencer
Godalming
01252 706767
Brass repairs and seller of "Snake Valve Oil"

   
Sharon McCallum
Cranbrook
01580 713901
Brass repairs and used trombone sales.
   
Paxmans
Linton House, 164 -180 Union Street, London SE1 0LH
020 7620 2077
The French Horn specialist for student and professional instruments and for repairs.
 
   
Phil Parkers
106a Crawford Street, London W1H 2HZ
0207 486 8206
London's best know brass specialist for student and professional instruments and repairs.

Manlio Mannisi
Ashtead
07840 069921
Woodwind and brass repairs

See also http://www.namir.org.uk/memberlist.htm for more names.