Walmart sells ws fluid if you wanted cheap stuff to flush with and then fill up with toyota stuff. I'm more of a drain and fill guy myself but you should be fine.
Which fluid do they sell that's WS compatible?
Edit: Quick online check shows this:
Super Tech DEXRON VI Automatic Transmission Fluid is specially formulated with premium base stocks and advanced additive technology.
Super Tech DEXRON VI Automatic Transmission Fluid, 1 qt:
Specially formulated with premium base stocks and advanced additive technology
Officially licensed and GM-approved
Meets the requirements of General Motors automatic transmissions calling for DEXRON VI fluid
Suitable for use where DEXRON II, DEXRON III, JASO M315 (1A), Toyota WS and Toyota T-IV fluids are recommended
Protection against wear and transmission shudder
Resistant to thickening due to high temperatures
Provides friction stability to maintain smooth and consistent shift performance
Seems pretty broad. Not sure if I'd rather have this as a residual in my tranny or 47,000 mile factory fill WS.
Update: Turns out my local dealership is selling quarts for $7 each, no matter if you buy a case or a single bottle so I picked up three to have 15 on hand. Since this is the first change I'm dropping the pan, changing the filter and then flushing it. I just dropped the fluid and grabbed a sample to send in to Blackstone.