Fatwallet is a good source, though a lot of the Dell coupon code offers over there aren't unfortunately being offered for free (though paying a nominal $15-$50 may be worthwhile for a 15% discount off an M60).
Another forum that jyoung pointed out as worth checking out is: http://forums.dealcatcher.com/tt.asp?appid=0
(click on the "One Time Use Dell Coupons - Post here" thread)
Note that you may be able to "stack" 2 or more coupons (of different types) for an even more substantial savings.
Dell comes out with a new round of discount/rebate plans every Wed midnight...also you may have to exercise considerable patience before someone will post or private message the desired coupon codes to you...really just a matter of luck & timing (I basically waited about a week before managing to snag a total of 20% off, plus some other instant rebates).
Don't forget, Dell has a very liberal 30-days full-refund policy on their notebook systems (no restocking fee, though you'll have to pay for the return shipping fees) , so if the new Sagers or something nifty come out before then, you can always swap...
Best of luck Hrana!