Out of curiosity have you tried reaching out to Deltec USA support and see if they have a source? TSA seems like one distributor - could also email them tonight so they get it in the morning. I think you already tried BRS but maybe try their support bot and see if they answer during working hours tomorrow. If you do so now it will send an email. I've had reasonable turn around using that as well.
I do not have the skimmer so do not know the actual tubing size id/od. Another source is amazon. I would reach out to Deltec first though to confirm the actual size required. It could be as simple as searching after that. I did to see out of curiosity and Plastic 3/8" Hose Barb Tee 3 Way T Shape Barbed Fitting for Ozone Water Oil (Pack of 6) came up.
I am not suggesting to buy that. But if Deltec or TSA or BRS reply to the actual size needed it may be easier.
Apologies that I don't have an exact source or fitting size.