https://www.bitsandkits.co.uk/ - reliable if sometimes a bit slow, wide selection but often goes out of stock of big chunks of it.
http://www.bitzbox.co.uk/ - smaller selection, only ordered from them once but it arrived fairly quickly.
https://www.bitsbay.it/en/ - really big selection, but slooooooow, maybe to do with their local postal service? Still it arrives eventually, useful for when the other sites don't have something but you don't need it quickly.
http://www.bitzarium.com/en - just ordered from them for the first time, got a dispatch notice within 24 hours.
http://www.forbiddengaming.co.uk/ - reasonable selection, ordered a couple of times from them and it was decent, not super quick to ship & arrive but not properly slow either.
Beyond those, ebay's your best bet really. There might be more local sellers for you but I didn't notice one when I went looking for the above a while ago.