Below is an EXCELLENT reference table for the different scales and sizes of miniatures … it includes both ratios (i.e. 1/300 or 1:300) and heights (i.e. 5.4mm).
http://www.theminiaturespage.com/ref/scales.html
A&A Scales (size listed first is best):
Ships - 1/3000 (.54mm) or 1/2400 (.67mm)
Armour - 1/300 (5.4mm) or 1/285 (5.6mm)
Aircraft - 1/432 (3.7mm) or 1/300 (5.4mm)
Infantry - 1/76 (21.2mm) or 1/72 (22.4mm)
Here are a couple of GREAT links that I’ve found:
http://www.pfc-cinc.shoppingcartsplus.com/page/page/292434.htm
http://www-2.cs.cmu.edu/~tpope/miniatures/pewtercraft/index.html
http://www.ghqmodels.com/M/catalog.php3
http://www.toysoldierco.com/Default.htm
http://www.warweb.com/miniatures/qualitymod.html
Also, extra playing pieces and expansion sets for A&A can be bought at the following sites:
http://www.xenogames.com
http://www.eplanetx.com/boardgames.htm
http://www.gamepart.com/product/item/fly_101.html
http://www.tabletactics.com/products.html
http://thegameshop.ca/enonhoraaex.html
http://www.impgames.com
http://www.dozensofgames.com/foncolalsec.html
http://www.andale.com/stores/sf_home.jsp?mode=1&sfUrl=strafezone
In addition, you can often find extra playing pieces and miniatures on eBay for sale.
Hope this helps!