We had issues trying to find a canister on STT this past summer on a charter for a crew member that is a non-swimmer. We were able to find a local supplier (the second one we tried) who theoretically had the one we needed. As they were going to the area of the charter company, they kindly dropped it off there for us. The ones that arrived had the incorrect thread and were not usable. If starting on Tortola, I would suggest checking with local suppliers to see if they have the one that you need and arranging pickup or delivery.

We try to eliminate checked baggage so checking it was not an option, though according to AA at the time, acceptance of the canister in checked baggage was on a case by case basis. Similarly, UPS rules I recall were ambiguous for shipping in advance.

The other option (costly) if necessary is to buy one on island.