How to determine for sure location of tire leak?

2007 Odyssey.

Front Left tire loses about 5psi a week, maybe more large temperatre swings.  Front Right is about half that.

I couldn't find a leak in the front left with soapy water spray.  I asked the dealer to fix the front left.  They couldn't find a leak.  They pulled the bead and the valve-stem but couldn't find the source.  

This car will need new tires soon.  I want to make sure that this isn't  a wheel problem before I buy new tires (because if it is, I'll get a wheel/tire package instead).  Is there an easy way to do this?  I do not have a container large enough to submerge the tire/wheel.

Aluminum wheels of a certain age and corrosion will do that.  The cure that I've seen it for the tire guy to dismount the tire, leave the wheel on the machine and run a wire wheel angle grinder abound the bead area a few times, then remount the tire with better bead sealer.

My front tires have been doing the same thing and I've been too lazy to take them back to the place that mounted them.

You can get a large plastic tub at wally world for not much.  Look in households for a storage tub. When you're done, use to to store crap in the garage.  Win-win.

Green Stuff in a spray bottle, soak it down well and wait at least five minutes.  Look for the fuzz.

I worked at a tire shop many years ago and we had a big water tank that had an air powered ram to hold the tire completely submerged.  It made it easy to pinpoint even the smallest leaks. 

My '06 F150 would lose air pressure sooooo quickly in sub freezing temps. Especially in negative temps. I brought it into commercial/fleet Pomps Tire and they removed the wheels, unmounted the tires, ground down the corrosion on the 20s, and slapped it all back together for something like $125. That was an easy bill to pay especially after not sitting at the gas station for 10 minutes to fill up four 33" tall tires in -15°F weather.

50/50 dawn and water. Douse liberally. Where dem bubbles at?

The tire shop I usually go to uses a 2/3-barrel on its side full of soapy water, for everything up to big rig tires. If it's hard to find, it's probably a dirty bead seal.