Gday Sean and welcome, good stuff helpin out your dad get back into it!
A few places I have used and can recommend:
single cards
checkoutmycards.com
ebay of course
other forums (may be able to pick up some bargains or trade)
boxes
cherrycollectables (also does live box breaking on ustream on monday nights, member here and website same name)
thisisselling (member here and sells on ebay)
westriot (member here and sells in ebay)
bbcexchange.com (baseball card exchange - but has HEAPS of old school boxes at good prices, especially worth the postage cost if getting a few boxes)
dacardworld.com (great range and ok prices)
hope this helps