+1 as to what was mentioned before: if you want something specific and you want it now and you're willing to pay the price; your LFS or, primarily, online retailers are the way to go.
If you are patient and can wait for a piece to come around then local clubs and fellow reefers are the more cost effective route to take.
Online retailers, IME, are far more diverse than your typical brick and mortar shop (LFS). There are exceptions to the rule, but most run of the mill LFS's simply don't have the means to stock every coral under the sun in their store. Trying to accommodate everyones needs is unrealistic and in most cases just not practical. This is also relevant to husbandry and skill set. I've been in some LFS's that had the know how to keep a wide array of very nice coral and some that simply did not...
Online retailers have limitations as well, but most are becoming very competitive in shipping rates and many more reefers are getting in on group buys. Some online retailers likely have less over head and in turn can stock a broader selection; not to mention they touch a larger, wider, variety of customer with an online presence which opens up their market and in turn their selections tend to be broader.
In regards to cost, IME this is absolutely relative to your location/area, as it relates to local trading or LFS's. If your area has a high volume of coral A, naturally the cost will go down as coral A is highly abundant. Conversely, coral A might be two or three times as expensive a few hours up the road. Naturally, prices vary far less for online retailers in regards to location. The only price varying I see from online retailer A and B is directly related to "name brand" corals or site reputation. A place like battle corals,
WWC, Unique Corals, etc are "higher end" comparatively to something like aquarium depot, live aquaria, etc. That being said, you also get what you pay for. Unknown Acro A: $7 VS Super Duper Fairy Unicorn Princess Acro: (1) Arm.
... .02