mp3 wise : beatport.com breakbeat.co.uk dogsonacid.com and most online records stores stock their fare share of mp3s.
don't know about tape packs but i always found juno and chemical the best for vynil, try http://www.tapepacksuperstore.co.uk/ i never used them but looks like they got a good selection
yea mp3: trackitdown.net beatport drumnbass arena for vinyl, 'bm soho', 'hard to find records', 'knowhow records'.... all great shops with really good websites. try and make it 2 the actual shops tho, especially bm...cos u'll never get hold of exclusive promos on the internet.