I've always loved that element to Pinkie - her ability to summon the craziest machines out of nowhere
Remember, though, that Pinkie Pie is friends with pretty much everyone in Ponyville because she's that sociable and full of vigour. Someone either lent her a helicopter, or she built it herself. I'd like to think it be the latter, though; she always has the craziest ideas and I totally wouldn't put it past her
The money would come from Sugarcube Corner income and resourcefulness. If you look at her helicopter, it's built out of springs, candy canes and flowers. And if you remember S1E24, she has a lot of random stuff lying around, so she'll have constructed something out of spare parts without needing to spend anything.
A small headcanon I had (I might still have it, in fact... it was from last June) was that Pinkie is actually secretly loaded from an exterior source of income, but she keeps it secret from everyone so that they don't feel inferior. Again, wouldn't put it past someone as caring as Pinks