It is perfectly normal for the wheels (especially the rear wheels) of most DSMs to be tilted 'inwards' - that is, having the top of the wheels closer to the centerline than the bottom. This is the amount of camber that the car has stock, and is not a cause for concern.
For those with measureable excessive negative camber, there are several correction kits available from various vendors which involve plates, bolts, slotted struts or shorter control arms. Several DIY methods exist using off-the-shelf parts as well, meaning there is something available for both front and rear on 1G and 2G cars. Usually, these kits are not necessary unless the car has been lowered, in which case camber correction becomes a necessity.
Canadians will be happy to know that Canadian Tire sells the eccentric bolts (TRW part# 13251A) required to do the front camber fix. Americans can buy Ingalls Engineering bolts from NAPA. DSMers from both countries can investigate the Ingalls Engineering websitefor more information.
With regards to the control arm modifications, note that hardcore racers may prefer extending the upper arm, rather than shortening the lower arm, to prevent the rear wheels from being pulled in towards the centreline of the car. The difference is so slight, though, that most people can't tell the difference. Additionally, the upper arms need not be as strong as the lower arms, allowing the use of less beefy (and expensive) heim joints for adjustable upper arms. This is not a factor for solid welded arms. Brackets for extending the upper rear control used to be sold by Taboo Speed Shop but seem to be discontinued.
In August '99, Ingalls Engineering released a kit for adjusting rear camber on 1Gs using adjustable control arms. They also have a kit using brackets for the same task. Check their website for more information - it's the 3842 kit.
Several vendors also offer camber plates or other solutions for those wanting the ultimate in adjustability. For these, shop around the vendors page for Tiel, Carerra, Ground Control or other make camber plates. 1G owners be warned - they only fit the front.