Some of the step-up gacha deals have given rainbow dressblooms with a specific purchase.

8. Step-Up #4 (10/06 - 10/13): Dogwood + ten $5 bonus steps for possible rainbow skill/dressbloom
16. Step-Up #6 (12/2 - 12/9): Lycoris + $30 rainbow skill/dressbloom
20. Ardisia New Year (1/5 - 1/12): Ardisia only, 25% off ($45), free rainbow skill+dressbloom
22. Step-Up #8 (1/19 - 1/26): Victoria only, step 1 ($45) = single roll (standard rates) + rainbow skill/dressbloom, step 2 ($15) = Victoria
25. Step-Up #9 (2/9 - 2/16): $75 six steps, 1-2 random 5* + gold skillbloom and 1 6* + rainbow skill/dressbloom at the end
28. Random Step-Up #10 (2/23 - 3/2): Five steps, $48, 1 gold + skillbloom, 1 gold + skill/dressbloom, 1 rainbow + dressbloom

There was also a time when you could buy a rainbow dressbloom for... 3000 Nutaku gold, I think?

Those were limited time offers, but one will probably pop up again sooner or later.

Bear in mind that rainbow dressblooms are a LOT rarer on DMM, so they're fairly decently priced here.