My view from online stores i've used:
aquahuna - good for cheap fish and cheap shipping. Excellent warranty. Two caveat - frequently the fishes are fairly young so they will be fairly small; quite a few of the fishes might have minor defects. These are not critical to the health of the fish but normally such deformities would be culled. aquaticarts - ok but not spectacular sort of a compromise between aquahuna and wetspot.
wetspot - fantastic.
tangledupcichlid - interesting small store for less common cichlid - shipping expensive.
aquaticclarity - the place to go for apistogramma - shipping expensive.
ebay - depends on seller but if you are careful can be pretty rewarding.
aquabid - depends on the seller - i prefer ebay to aquabid
I really like the smaller stores but because they have low volume shipping is pricey esp since priority mail is no longer reliable they are forced to use overnight shipping.
If this is your first time using online store be sure you understand the DOA policy of the seller. Most places will only provide refund IF you take a picture BEFORE the bag is open (no exception). Many sellers do not refund shipping. Some places only provide a credit which means you will have to pay for shipping a second time. Some places require that the item be shipped again with you paying shipping. Be sure to read the policy and pick accordingly.