My Petsmart personally doesn't carry the medicated foods and neither does my Petco but small aquarium stores do. :/ Hrm. Skip that, just go with Maracyn. It's good for dropsy which is what he has if he's bloated. If I remember what I've learned from DarkMoon, then if you don't actually see the parasites, it's bacterial. That's what I was told when I rescued a betta with severe dropsy. So, IF you perchance find the medicated pellets, get the bacterial ones. Otherwise, Maracyn will work too.
Nope, you can medicate him in his little QT container. DarkMoon also gave me tips on that too. Get a big container, something you can put five cups of dechlorinated water in. Add in the medication and shake/mix well. Use 1/2 cup of this mixture per gallon of his QT container. So maybe if he's not in a gallon QT, get a gallon bowl or something? Save the mixture for water changes.
For meds I just take an old milk jug (1 gallon) and premix in that, since most meds can be mixed into a gallon/figured if it's for 10 that way. Then you can save it. It works wonders if you're QTing in a cup. You mix the meds in the really clean milk jug, and you just pour into the QT cup, and it's good for a couple days/water changes.
Uhm . . . how did I do this? Yes. I think so, yes. But if it's easier and you're more comfortable, by all means get a new 10g. Uhm, bit dishonest but you could just use the new 10g to measure out the medication, then clean it well and return it. I haven't actually done that, by the way. ^_^ Hint, if you've got a Petco near you, get your 10g there, they are having a dollar per gallon sale so the tank will only cost $10.
:P Or if you get a ten gallon you can always pamper him with a big tank, or divide later and get more fish!
elijahfeathers: yes, I like the way you think. omgitslph: use whichever dosing/measuring method you are most comfortable with. I know, they should really work on the dosing instructions instead of making them so generic for a 10g.