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Certainly a rental company that specializes would be the best way to go; but I can relate to budget concerns. Still you might consider shopping more places.
I do have a suggestion, but you still have to PURCHASE fabric...That alone will be costly, and then what to do with it after?
In theater, and in a rental company, draping is often done on "Pipe" . By that I mean upright pipe is used/ usually adjustable to various heights. Those pipes are set onto stands with a short vertcle pipe to hold the upright. Across a span, horizontally, another specific pipe is clipped into the two uprights; and one can slide draping fabric with a rod pocket sewn in, hence creating a drape. Multiples of those can then be used to hide walls etc.
You may get answers that suggest Cord??? You'll still have to support the weight; and the sag from the weight wherever not supported.
If I were you; I'd first measure off what you need. I'd probably RENT the runner. I'd price the amount of fabric,,,and check for availability; then compare all your costs as a DIY, and shop the rental houses.
Draping from a GYM ceiling seems overkill to me. I suspect anyone looking much above eye level at a wedding reception isn't truly "GETTING IT". Beyond that and with no offense meant, guests will already be aware of WHERE the reception is. Obviously it's for a very special occasion, and should be memorable, and hopefully ONCE in a lifetime; but the focus should be just as much on the event, as on the where, and how.