As far as aquarium plants go you really cat go wrong using this site. When I first started before i got introduced to this site I made a very large order from www.aquariumplants.com For larger orders the shipping is very reasonable but I have heard that if your just getting a few things it is quite high. Also http://www.drsfostersmith.com/ I have used for a handful of items they seem to have pretty good deals. And as you stated Amazon is also a good place to find odds and ends.
kensfish.com is my general go to place for supplies and such. petmountain is sometimes cheaper but their customer service and shipping is not up to par IMO.
had one issue with kens sending me excel instead of flourish. a quick email and the correct item was in the mail with an apology letter and no headaches.
20L My Other Tank: OEBT Shrimp 40B Inlaws Tank: 2 goldfish 75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry
I ordered from thatpetplace.com and petsolutions.com in the past. No issue and fast shipment over the years.
Like other online places, you need to sign up email to get special promotion prices and discounts.