The cheapest tyres are always the size with the greatest volume of sales, which these days is possibly 15". Just look around at how many Hondas, Toyotas, and other small cars are now running 15s. Definitely not 18s!
Here's a very cheap $195 set of Subaru 15" wheels on eBay at the moment (not the greatest in style but an upgrade from 14's):
http://www.ebay.com.au/itm/Subaru-Liber ... 0758453734Perhaps resprayed black or white (depending on your car color scheme) they might have some street cred.
Also found some black 16s here current bid is $50! (tyres would be a little more $$$)
http://www.ebay.com.au/itm/Subaru-fores ... 1099480996Good luck!