I didn't have enough money to buy one of those Halko Momoi 2009 Tour T-Shirts they were selling.
Is there a place online where I can buy one?
This is going to be tough if you do not know Japanese. Momoi's T-shirts are made by this Japanese Shirt Company by the name of Mars Sixteen as I read from her profile that she collects T-shirts from this Brand name as part of her Hobby.
Unless you know someone that can navigate through this website you are stuck looking around.
Anyway this is the website where her shirts were made.
http://www.mars16.com/
I noticed my Sunday Early Morning World Tour Haruko Momoi Shirt was made by Mars Sixteen since I checked the inside tag of the shirt.
Wish I could help you on this but this is all the info I know.
Now that Halko Momoi's coming back to FanimeCon, odds that these same shirts will be on sale again?
Most likely yes. Usually its after her panel or during her concert time. Try not to miss out as I have never seen a Seiyuu come back to Fanime more than Twice.
It's hard to know what shirts they'll have to sell; they may or may not the ones from last year (in the same available sizes). I'm sure they'll have some shirts for sale; hard to say what ones.