If you need a lot of regular flowers, you can just go on an island tour and pick up any flowers you see there.
There's also usually giveaways around here from people who are looking to get rid of their flowers, but it's probably fastest if you just use your island.
The Butterfly Catching tours have the best flower choices, in my opinion!
I think it might be the only one that has lilies, but I'm not 100% sure on that.
So yeah, unless you're looking specifically for hybrids I wouldn't waste your bells buying common flowers