I am searching for latex tubes to accommodate a 700x28 tire with at least a 60mm presta valve. I have not had much luck.

most won't have 60mm valves. vittoria makes a 25/28 mm but it has a 48 mm valve, you just need to use a 27-30 mm adaptor.

Removable valve stem and a valve extender. All you need. Most versatility and easiest to find. Vittorias are perfect.

I have some Michelins that have a 60mm valve, but I bought them a few years ago so don't know how readily available they are currently. If you can't find any with a few searches, the valve extender option is a good Plan B.

Michelin Latex tubes come in a 60mm valve size. Chain Reaction has them but probably available through US distributor as well. They are marked as 22 to 23 mm but I've used them for years in 28mm tires and for the past 4 months in a 30mm tire without problem.

Silca sells them with an extender, had no issues with them on my last set of deep wheels. The nice thing is that the extender is threaded, and when used with their speed shield you can get it secure with the nut, which is nice since you have to pump them up every ride. I wish the main valve was threaded, don't know why they make them smooth.

I would swear Vittoria and Silca had 60mm latex tubes available, but they sure don't seem to now.

Just get the Silca extenders. They work very well. I'd rather not need them either, but you can't get the 80mm stems in latex ever, so I've had to use them for a while.

Interesting.. you'd think the latex tube market (aka Marginal Gains seekers) would have a large constituency of consumers that are also running deeper aero rims.

This is 100% where my head was at.

You would, but I even asked Josh Poertner about it over on ST and he basically shot it down as a financial non-starter. That was before tubeless was big; I can't imagine the economics have gotten better.

^ This. They are quite a large tube and I have they don't actually fit in the stated tyre (22-23mm) very easily. Work perfectly well in a 28.

Offering longer valve stems on latex tubes is not done for financial reasons? Maybe Silca sells a lot of valve extenders and that's the financial reason?
Anyway, the reason Josh gives is something I'm not familiar with.. go to about the 3'35" mark in this video. He says the reason is that longer valves don't work in commonly available disc wheels (he's not referring to disc brakes, rather the solid disc TT wheels). I learned something new today :-)