There are heaps of places to get bits there Doc.

I find Ebay is often quite reasonable, but sometimes you do need to do some hunting around... For most electronic bits I usually try these Australian Online sites first...
RS components - http://au.rs-online.com/web/
Jaycar - https://www.jaycar.com.au/
Element14 - http://au.element14.com/

The two Ebay sellers below I have bought from have been quick delivery and have a reasonable variety of stuff. Think both have 5Y3's in stock..
Vicmazz - https://www.ebay.com.au/sch/vicmazz6...1&_ipg=&_from=
Amptubes - https://www.ebay.com.au/sch/amptubes...1&_ipg=&_from=

There are plenty of others (mostly Asian) that have similar and often cheaper stuff however often they only sell in quantities of 100 or more. That's way too many tube sockets for my needs... one site I got 3 Chinese made 12AX7/ECC83's for AU$32 which is pretty damn cheap.

I'm keen to build a EL84 amp... something akin to a Vox AC15... but that will have to wait until I get all my guitar builds done...