for coupon codes you can try retailmenot.com and plug in whatever store you want to buy from.
If you use Aquacave I can look up the Reef Central code. It gives you 5% off and shipping is free over $175
I just ordered a sulphur denitrator from Premium Aquatics and they had theirs listed significantly cheaper than I saw them elsewhere. Not sure why. Maybe their prices for RKs are good too. Haven't checked.
And then you can always check on Amazon and see what you find.
Or call Tom Tran at SoCalTropicalFishoutlet and ask about his best price (he may give you a coupon code over the phone)