Don't forget
Conjurer's Closet 
, it's a very good way to abuse Tyrant of Discord - it's 5 mana, but still cheap enough that it'll be ready to go before the Tyrant is.
Your best low-cost options for bringing white into your deck are probably things like
Evolving Wilds 
/
Terramorphic Expanse 
plus a few basic Plains to search for. Dual-lands that cover opposite colours are less common than allied-colour lands, but consider
Boros Garrison 
as a reasonably cheap option.
Sacred Foundry 
and or
Arid Mesa 
are the best bet, but they're relatively expensive.
You could also consider artifacts, as they also help you ramp into your Tyrant of Discord -
Boros Signet 
is the obvious one, also consider
Darksteel Ingot 
and
Gilded Lotus 
.
Hope this helps.
