An ingredient used as a foaming agent in both foods and in beauty products like shampoo and toothpaste is soap bark (Quillaja Saponaria). Note that this is different from soapwort (Saponaria Officinalis). 

This can be used as a foamer in cocktails in place of egg whites. 

Quillaja Saponaria can be found both in bark form or as a liquid extract. 

At Austin's The Roosevelt Room & The Eleanor, owner/operator Justin Lavenue uses the liquid extract but dilutes it with other ingredients to make it practical for the bar. (Note that they do not use this foamer exclusively - they use batched egg whites for egg white drinks.) He writes:

Our house foam tincture is something we add to a couple of Shaken-Up drinks in trace amounts (typically just one dash) to give it a small amount of head and give it a fuller mouthfeel. If you were to use three dashes of this tincture, it would produce a similar amount of head as a drink with full Egg White.

It's produced from Quillaja Saponaria, which is a highly concentrated sap from the Soapbark tree, and is used in many consumer packaged goods such as toothpaste to foaming hand soaps.

It's so potent, that we dilute ours 16:1 with a Water & Vodka solution, and even at this concentration, 1 dash is all it takes to make a noticeable impact on a drinks top layer of foam and its body. Unfortunately, the sap in its raw form is extremely tannic, so just diluting it with this Water & Vodka solution leaves drinks out of balance. To counteract this, we balance the tincture out with a little bit of Sugar and Salt.

Here's his formulation, which certainly must last a long time:

"RR FOAM TINCTURE" - QUILLAJA SAPONARIA EXTRACT TINCTURE (16:1)
From Justin Lavenue of The Roosevelt Room (Austin)

Yield: Approximately 8.6 Liters (290 oz)

Ingredients

One 16 oz bottle of Quillaja Saponaria Extract

120 oz Cold Water

4 Liters (136 oz) Vodka

10 oz (by volume) Sugar

20 oz (by volume) fine grain Salt

Procedure

1. In a Large Cambro, pour in the Water, Vodka, Salt, and Sugar, then stir well to dissolve the Sugar and Salt.
2. Very gently pour in the Quillaja Saponaria Extract.
3. Stir very gently to mix the ingredients together.
4. Gently strain into twelve 750ml bottles.
5. LABEL AS “FOAM TINCTURE” & DATE CLEARLY, and store in a cool, dry place.