Your brokerage account agreement (or perhaps your broker's website) ought to have some text describing how disputes get resolved, and hopefully also give you some idea of what their obligations were in respect of this fill.
Sometimes the dispute resolution mechanism allows for mediation by a neutral third party (I think its NASD or similar in case of stocks, other bodies in case of futures, options and forex).
If you decide that you have good cause for a claim, find out about the mediation process, and start documenting your claim.
Get the names of the individuals at the brokerage who dealt with you in this instance.
As soon as the brokerage sees that you intend to cause them trouble in this way, they'll probably cave in ... particularly if you are the sort of trader who pays them more than $1000 commissions every few months!
Good luck.