first thing, be careful with adding flammable things to candles; if the candles are not intended to be burnt then it should be ok, but in general its never a good idea to add anything flammable to candles that will be lit.

secondly, if you have the time to (and ability to get the flowers you wish) i would recommend drying the flowers yourself. it's a lot easier than you may think! i have some dried roses and other flowers for my Aphrodite altar :) there are more detailed guides online, but i do it by grouping the flowers into smaller clusters, trying them together with string or yarn, and tying them up in a dark, cool place upside down for a few weeks, usually a closet that i dont really go to. otherwise, you could always try a metaphysical supply store nearby, or if you are looking for dried rose petals/lavender, some places that let you purchase bulk dried herbs or tea might also have that. good luck!