We sell our stock of tube couplers at outrageous prices. Well, double-ish, anyway.
You may be better off buying directly from our source. They have lots of options.
https://www.usplastic.com/
It all depends on the number you want, shipping costs and convenience. If you are buying a board from us the shipping is flat rate so adding on the connectors may make sense.
These Luer connectors work. You can mix and match the sizes.
Here’s what we use:
https://www.usplastic.com/catalog/item.aspx?sku=64273&catid=551
https://www.usplastic.com/catalog/item.aspx?sku=64275&catid=551
https://www.usplastic.com/catalog/item.aspx?sku=64368&catid=551
https://www.usplastic.com/catalog/item.aspx?sku=64370&catid=551