I think I finally found an equal. Polymer Acrylic Varnish from Golden. It smells just like the GW washes, and like I always say, follow your nose. The best thing about it is you can mix whatever color of wash you want. My version also costs less than $1 per bottle.
First I started off by trying to mix my own versions of Agrax Earthshade and Nuln Oil. Here are some of the materials:
A bottle of Polymer Varnish with UVLS (Satin) from Golden Artist Colors, and acrylic inks from Liquitex and FW (Daler Rowney). The inks I used are Transparent Burnt Umber and Black.
You can find acrylic inks at many art supply stores (Hobby Lobby in the US carries them). Golden products can be found at higher-end art supply stores, such as Utrecht or Blick.
I cleaned out an old wash container, and filled it up about 3/4 of the way with varnish. The amount of ink you need depends on how much varnish there is and how strong of a wash you want. Add a few drops at a time, mix, and test it out on your palette before adding more. A little can go a long way!
The pot above is my mixture of Nuln Oil. If I recall correctly, it's about 5-6 drops of black ink (which is pretty strong) and 1-2 drops of Burnt Umber.
Finally, add some clean water, maybe 20 drops from an eyedropper (I have a couple of old P3 ink containers, which I cleaned out and fill with water. They make great eyedroppers)
Above is my version of Agrax Earthshade next to the real thing. Can you tell which is which?
I believe I did about 10 drops of Burnt Umber, 3-4 drops of black ink, and 1-2 drops of red ink (old GW ink).
You can make different color washes too! The beard above was painted with a dark grey wash.
Basecoat VGC Stonewall Grey. Shade with a mixture of Stonewall Grey and the black wash mixture. (I like to add a little bit more varnish and water. This makes a nice grey wash, great for shading over light basecoats). Once the wash dries, highlight with Stonewall Grey, followed by layers all the way up to white.
The gold on this slayer was given a burnt orange wash of Yellow Ink, Dark Flesh, and Polymer Varnish. The gold was then highlighted up to Mithril Silver, and was finally given a very light glaze of the original wash. This makes the highlights look less pale and more yellow.
I hope this information helps you discover some new washes, saves you a bit of money, and more importantly boosts your painting up to the next level.