I use Care-A-Lot pet supply for big things (crates and gates): http://www.carealotpets.com/ as their prices are pretty good, and I’m fortunately within one or two day’s shipping time from them…
Sitstay has good stuff, too: http://www.sitstay.com/ with free shipping (on most stuff) over $75 and Good Dog Discount Days on Tuesday. (Their boxes are VERY cute, too!)
KV Vet is an old standby (free shipping on anything under 5# as long as you spend $50 or $60): http://www.kvsupply.com/KVVet/assets/html/homepage.asp?URLCheck=1
And SmartPak (of course), especially when I’m ordering big or irregularly shaped things - the free or flat rate shipping is wonderful!
White Dog Bone: http://www.whitedogbone.com/index.html is good, too - I usually see them “in person” at dog shows, so I can’t really speak to the website, but they carry good stuff at good prices at shows!