> If I want to buy BTC from her (by physically handing over the cash), what service providers do we need to use?
This is the easiest to answer so I'll start here. You don't need any service providers beyond:
1) Any one of the many free bitcoin clients out there to
a) create a wallet for you, and
b) send the bitcoin to that wallet
In the article, the author wanted to trade stocks on TradeZero and he wanted to send TradeZero $5-6k to open a new account. TradeZero provided two options to send that cash:
1) Wire it from the bank
2) Convert your dollars to bitcoin, send that bitcoin to BitPay, which will send it to WB1, which will send it to TradeZero. Oh, and everyone will take a fee out in the process.
My suggested alternative, is that if the author already has bitcoin, is to find a stock-trading company that will just let him fund the account with bitcoin. Then the options would be:
1) Wire cash from the bank
2) Send some bitcoin to the company's bitcoin wallet
In that case, the fees incurred from 1 would be $25-50, and the fees incurred from 2 would be approximately 5 cents.
Given all that, I have no idea if there exists even a single stock brokerage that just accepts bitcoin directly, so that likely isn't a real option.
