Yeah, it's easiest just to provide the wardrobe. I believe in the BC Performers Union contract, you can require actors to provide one wardrobe 'outfit,' but you must provide any additional wardrobe required. And if you do make them provide part of the wardrobe, you are responsible for any cleaning/repairs required.
Most of us will if you need us in standard clothing but if it is supposed to be a uniform... not everyone has those. If you are doing a 'color theme', make sure to tell them what those colors are too. and ditto what 4100 said about cleaning and repairs.
As an actor, I don't like playing parts where I wear my own wardrobe. It's like I'm playing myself, which annoys me to some degree. I'll go out of my way and buy an outfit, which is sort of unfair since it accounts for so much real estate on your film.
If you're going to ruin the costumes, or at least get them dirty, I advise you to have a fun day where everyone goes to a thrift store to pick out what their character would wear. (it's always a good idea to buy 2 or more for clothing you plan to ruin)
Otherwise, try to inspire them in some way to purchase these outfits on their own dime.
I've run a number of low budget shorts that involved fake blood on clothes.
In order to guarantee a possibility of multiple takes and/or reshoots I bought several
of the same article of clothing that would be bloodied. This can be done cheaply if you
plan carefully. It's most important that when camera's are rolling and its crunch time that
you have spares of the same clothes and options.