Perhaps you can do a google image search. Good luck!

I have had trouble, in the past, with matching tubes and stoppers that are not available from the usual suppliers. However, I discovered a small, UK company, that has been very helpful in sourcing sample tubs, tube stoppers and cuvette racks to fit our requirements. They sent samples, for us to check, in the case of stoppers, to ensure a good fit prior to purchase, and have supplied to international customers in the past.

You could try this item:-

https://www.plastictesttubes.co.uk/5ml-graduated-flat-base-ps-test-tube-16mm--x-60mm-4504-p.asp

But the following tube caps might be more similar to the ones that you have been using, than those shown to go with the 5ml tubes above:-

https://www.plastictesttubes.co.uk/stoppers-for-16-17-mm-tubes-7-p.asp.

This company also sells on Amazon and E-bay, but it is better to e-mail their website, as their Director may be able to source other items for you, if you are seriously intending to purchase.

I hope this helps.

Zoë.