"Bow shop guy said mine can't use the less expensive type."

Did you ask why?

I use $15 arrows for accuracy and $5 arrows for fun.
With a Browning 6T6L bow.

I didn't listen too well, but i think it's so the arrow will go faster, since i'm only shooting 40 lb draw weight. It is a thin, light arrow. Not sure how much it weighs, but the broadheads are 75 grain.

I don't necessarily believe everything those guys say, because the same guy told me to point the bow toward the sky and draw back and it'd be easier to draw. Sure, it is, but every thing i've read says that if you have to point it at the sky, it is too heavy for you. So, he could be wrong about the arrows, too.

Plinker

He is wrong! Only trying to get you to anty-up some hard earned money.
Find out the number or the carbons and then go to the Easton site(search on Easton Archery) and look at their arrow shaft calculator and select an inexpensive aluminum shaft that matches the spine of the carbon and you should be good to go. Match the spine and weight of the arrow as close as possible and you have some inexpensive 25 - 30 a dozen blunt/practice arrows for stump shooting.
Also as another thought, If you are too concerned with loosing or breaking the most important and "inexpensive" part (the arrow) then how are you "SURE" you can make that 30 yard shot in the brush? I practice knowing in the corse of a summer I will go through 2 dozen shafts lost or damaged by shooting through little lanes and over and under branches ect. My thought has always been it is cheap to loose or break them in practice so I KNOW what the arrow flight is like. This way when I am in the woods and that trophy is standing behined a forked tree with his head hidden, I KNOW whether or not I can thread the needle of the fork of the tree into his boiler room.
I know arrows are expensive, but it is a cost that I feel is just like practicing with your rifle or handgun. You don't shoot a 22lr all summer when you are taking a 300 win mag into the woods for Elk or Moose.

Plinker: Definitely do not shoot at the dog with a blunt. When the dog gets home, his owner will be pissed about the arrow sticking in his dog.

I won't shoot blunts at all. Field tips are OK for stump shooting, but blunts destroy the arrows.