Check out agriculture supply stores for water containers. Sometimes they have models that aren't too pricey. A local store here has 145 gallon upright containers for $140 IIRC, and they have a little stand you can buy for another $40 so you could gravity feed to the sump. They are only 28" wide, so they fit down stairs, through doorways, etc. See if you can find something like these. You have to clean them with a long handled scrub brush, but I think their small footprint is worth it.
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