You will need to find an older version of ROM Manager and sideload it and install it. Unfortunately, the current version of ROM Manager requires Android 2.2 and up, which is why you are probably having difficulty finding it in the Android Market.
I say Android Market because the stock Sprint HTC Hero ROM is Android 2.1 which does not support the Google Play Store. Once you flash a custom recovery, such as ClockworkMod Recovery, you can flash an Android 2.3 ROM and get the Google Play Store.
Nokia 638 > Motorola Talkabout > Motorola V60t > Motorola V60i > Motorola V505 > Samsung Epix > HTC Hero
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