You don't need to buy them!! they're native to the UK!! They breed, well like flies.
Lady-birds are also excellent at controlling aphids.
If you care about your garden, you wouldn't encourage butterflies per-se. The common cabbage white, for example, is an extremely destructive creature, and will decimate your greens in a matter of days, plus you're unlikely to gain any friends in your neighbourhood, if they can trace them back to you.!!
<<<<<<<ADDENDUM>>>>>>>
Beware of releasing any creature in an 'ad-hoc' fashion, you may well fall foul of the rules laid down by DEFRA and the EA (Environment Agency) :-
You are not permitted to release any species unless it is indigenous to the UK, and the area/environment into which you are releasing it.
You are absolutely forbidden to release any species that is not indigenous to the UK, or has been imported.
You are not permitted to release any species if you live in an an area of SSI (specific scientific interest). You must contact the EA to ascertain the status of your area.
You will be prosecuted by either DEFRA or the EA if, by releasing any species, you cause harm, directly or indirectly, to the environment or to the legitimate businesses operating in such environment.
For example, if you release an insect that goes on to kill an indigenous species (direct), or contaminates another species, ie, an insect, that bites a cow/pig/sheep/horse etc. and infects such species with any disease that cause harm or death (indirect).
Beware, you cannot just release an insect "because it takes your fancy".