It is a selective preemergent herbicide for certain broadleaf weeds and annual grasses in landscape ornamentals, gound cover, container grown ornamentals ect. You should be able to get it at any supplier. You can use a broadcast spreader, drop spreader, or hand shaker. It will kill annuals. So if your flowers are annuals don't use it. My supplier says the first year you can expect 75% results. And 95% the 2nd year.
preen or snapshot would do the trick....there basically made up of alot of the same chemicals..and do about the same job..
if there is no desirable plan tlife in the mulch bed then i would say hit the weeds hard with round-up before you install the mulch. Just be carefull you dont go stepping the the round-up then decide you want to go grab a pop from the truck or something and go walking across there lawn....then you will really have a pissed off customer.