For amp techs in the UK here is some useful resources.
Electronic Components in the UK
For small volume parts try bitsbox.co.uk first and plenty of ebay sellers are ok too. reasonable prices and fast shipping.
For passive parts like resistors and caps not much risk of seconds or fakes but there is a reputation of this on online marketplaces.
Large scale uk suppliers include
CPC good range of parts £20 free shipping
Rapid online similar to CPC but slightly less range.
Farnell do larger volumes generally and their shipping has got too expensive to be usable. They used to offer free shipping and now it is a tenner.
RS components are similar to Farnell but they also have a few retail stores dotted around the country. They tend to aim for larger quantities.
Mouser is US based but their range is impressive and shipping is free over about £30
Digikey similar to mouser
For Valves options include HotRox, Karltone and Watford Valves.