I understand your frustration.
If I was in the same situation I'd be pretty upset too.
It's not the fact that she won't buy you a fursuit but rather the reasoning behind it.
Just know I'm here to provide an insight for you as best I can.
As HollowOfHaze suggested, you might want to think about investing in a fursuit further down the road.
It's a sad truth to hear, especially while you are so excited to get a fursuit, but always ask those who've gone before you.
In other words, there are many reasons why people may suggest certain things to you and it's usually because they have learned from experience themselves. I'm sure you're aware that suits are expensive but at an early age they can be even more "costly" because you'd eventually grow out of one and have to pay for a new one all over again!
Ugh, I hate that fact, but it's true. So you're going to have to bite your tongue for now..
But I say you may be able to look into alternatives! I mean yeah, you can't afford a fursuit now but.. what about cheaper methods of expressing your furry side? I've got a nice white wolf tail that I occasionally walk around with. I hook it to my back belt loop and it sways around seamlessly! You wouldn't believe the amount of compliments I've received for wearing the thing around town, seriously!
How about ears even? they're adorable and I'm sure you can find some online. Google is your friend!
So all of that is entirely up to you but now I want to speak to you about the concern your Mother has for the Furry fandom.
She has every right to be worried. I cannot condone what certain TV shows portray us as but you must keep in mind that it's a parent/guardian's duty to protect their kids. Sometimes a parent's reasoning behind their actions seems a bit unnecessary but through experience I've learned that they are trying their best. So try and not get upset at her for not understanding what you're into but give it time, she eventually will. I know it took me a couple years to convince my family that being a furry is not what they thought it was.
I did research, I showed them videos and even had to explain in my own words what being a "Furry" is all about.
So in conclusion, have some patience and look into alternative ways you can exercise your furry side.
And maybe show her some videos explaining what furrys are. Sometimes it takes a third party opinion to sway someone's outlook.
Cheers