Anyone with a spare flat A-string?

[HogieWan]

I just got a cello to learn to play, but I got a 5-string cello. However the string that was supposed to the low-F just wasn't thick enough and it has no tension tuned down that low. I thought maybe the strings on it were supposed to be set up with a high-E instead, but the A-string popped when trying to get it up that high.

The only low-F I can find is a D'Addario Helicore made for the NS electrics, but it cost $40 for one string. NS says that a thick gauged flatwound bass string would work as a substitute.

Does anyone have a spare? Anyone know where I can order a single real cheap?

Thanks in advance