RVW as the best deal when I was buying. I've heard good things about PPL, too. I use etrailer for all of my general towing needs, but WDH kits do not tend to be priced well by them.
RVW typically keeps full kits in-stock, and therefore deeply discounted. Chances are, a full kit (with a shank) might be cheaper than a kit without the shank. By whatever is cheapest. If you need another shank, etrailer will have it cheap, and you can re-sell the shank you don't need.
That's what I did several years ago (although I'm so lazy, the original shank is still sitting in my garage, in a box, unused).
Anyone want to buy an unused Equal-I-zer shank, from the standard 10k kit?
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller