I like KV Supply, Jeffers, Drs. Foster & Smith, and PetEdge. There is also 1800PetMeds. Chewy, PetFlow, and a slew of others that I can't think of at the moment. You can also get pet supplies on Amazon and eBay.
You can post a wanted ad on Freecycle. I get gardening supplies that way. We get meds and big ticket items like beds from Drs. Foster & Smith because you want them to last. Geartrade.com often has specialty dog stuff for outdoor sports.
For collars pet smart has a pretty cool Marvel comic line, like the hulk and batman and who doesnt love an iron man collar??? lol
Also, etsy has a funny farm boutique shop, i like the collars they have, very cool "nom nom sushi" is my fave!
eBay has the cheapest crates and they're just as good of quality as the Midwest ones you'll find at your local box store. You can also pick from a variety of sizes as although there are standard lengths of 32, 36, 40, 48, they vary a little in height and width. You can find something that will fit better if you need to put it in a certain space.
I'm usually cautious about buying stuff for my dog off Craigslist because you never know what happened to the other person's dog. Things like parvo and other diseases tend to live on things for a while so its not worth it for me to save $20 for something like a crate. I will say that I'll buy pretty much anything else off of craigslist...