Most caps are pretty much universal. You just get one the correct size (5/50) and the correct voltage rating (370 VAC or higher). They can be either oval or cylindrical in shape. As to the cost, the last one I bought (myself, not through a contractor) was ~$20. In fact, the one that I bought is nearly new and I don't need it any more (just installed a new heat pump). If the ratings are correct, I'd be happy to send to you. I can't remember if mine is 5/35 or 5/50. I do know it is 370 VAC. I'll check tonight and let you know.
They are pretty easy to change. Make sure there is no power to the unit (pull the disconnect) and take a couple pics of it before you do anything (so you don't forget how the wires go). Slide the wires off the old cap and slide on the new one. Then mount the new cap in the holder. Probably only take you 10 minutes to do the whole thing. However, I would probably just keep the new one on hand and wait until it is needed. The other option is to switch them and keep the old as a spare. This way if the new one blows, you won't be stuck on a really hot/cold day waiting for a part or paying big $$$ on an emergency call to get someone to fix it.