Well there are a few things you can do and I will try to answer each one as best I can.
Couple of last things. Have you decided how you will charge people for their submissions into the auction? Percentage of sale or flat fee? Also when (and if) you get to sales of donated items be sure to pump the store or individual that donated the item. i.e. "This great specimin of a (enter fish/plant name here) was donated by XYZ Fish Store and the proceeds will be going towards covering the expenses of this auction and our annual expenses. We thank XYZ for their generous donation. Now who would like to start the bidding off at......
Don't hire one... find one of your more out going members that is willing to do this for you. I would bet that a professional auctioneer would be expensive and more than unecessary. Furthermore if you do hire one all the proceeds of your auction will go to pay them and not the club, counter-productive to why you are holding an auction... correct?
You have mentioned the bigger three. Have you also thought of notifying all the local aquatic societies around your club? They may have members that might be willing to travel. Furthermore you will need to bait them. So let out a partial list of what is up for sale in your auction. Sometimes the right fish will bring them out. When notifying them of your auction be sure to mention a quid pro quo on the notices. They will be more willing to assist advertising yours if you agree to advertise theirs. Furthermore have you thought of advertising in the Aquarium magazines? I am not sure if they charge for the classified at the back but there are a number of them and they are read buy quite a few hobbiests.
Many LFS are willing to assist with auction by means of donating fish or plants. Just don't expect them to donate anything rare or exotic. I do know that one of the LFS in my area frequently donates food to the society which we give away at meetings to all paid members in attendance. Furthermore offering them a table to display their wares and make some sales is effective advertising for them as well as assists them in recouping the cost of the donated fish. If they are unwilling to donate anything to the auction I would charge them a table rental.
You will need a cash float to help make change with and someone to man that table. Ask your treasurer as you already trust them with the money of your society. If you can also have a cylinder of O2 on hand for those people that are attending and have a long trip home. Make sure you also have a supply of water treatment liquid as sometimes bags break and they need to be transferred to another bag. The treatment will help with the stress of the fish. Lastly don't forget to have membership forms on hand and someone to answer question of any prospective new members. Auctions can be a good source of memberships.
Couple of last things. Have you decided how you will charge people for their submissions into the auction? Percentage of sale or flat fee? Also when (and if) you get to sales of donated items be sure to pump the store or individual that donated the item. i.e. "This great specimin of a (enter fish/plant name here) was donated by XYZ Fish Store and the proceeds will be going towards covering the expenses of this auction and our annual expenses. We thank XYZ for their generous donation. Now who would like to start the bidding off at......