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.