I've heard that Blair spray damar varnish and Grumbacher liquid damar varnish are among the best and are worth the price. Make sure you buy the size that you need so you can optimize the usage and minimize wastage.
Have you tried making your own damar varnish. It is pretty easy and very cost effective. You need a little damar crystals (cost: about 10 $ a pound) and some pure gum spirits of turpentine.
You can drop into any paint store and actually compare the quality of the varnishes available and ask the shop keeper for an idea. Alternately check online in some paint and product stores which offer comparison of various brands and also will offer user reviews of the products.