the milkweed you buy at Home Depot, Lowe's, etc. isn't native milkweed and won't be available until next spring. That kind of milkweed is tropical and the monarch's do love it more than the native variety. but it must be cut back a lot during the season as over time it becomes a source of a bad parasite that causes deformed butterflies. here is a website that gives good information:
Monarchs and Milkweed here's also a site for free seeds (they like a donation) so you could start growing plants now:
Free Milkweed Seeds & contributions for Live Monarch Foundation- Get your milkweed seeds for the Monarch Migration