I just recently completed a final gear swap in my FR-S a couple days ago and have another thread going on that. What I want to do here is give the recipe I followed all in one place.

For reference, when I refer to a "carrier", I mean specifically the whole pumpkin assembly with the actual differential installed in it. Carrier, pumpkin, third member, same thing.

Note for turbo guys: If you're looking to swap to a taller gear ratio, just go get yourself a carrier from an IS300. You want either an M98 (3.9 final drive) or M85 (3.7) code. These are the LSD variants for the automatic and manual IS300, respectively. Both of these are a direct swap into our cars, the only trick is to pop out the axle stubs (the wheels on the sides) as our axles simply spline straight into the carrier. You still may consider overhauling the carrier/differential depending on the condition yours is in, but otherwise you can stop reading here.

OPTIONAL - Donor Carrier
What I did to minimize my downtime was to obtain one of the carriers mentioned in the note above, and had my gears installed in that. Added bonus of having a second working carrier in case something goes Horribly Wrong.

Optional Cost: ~$300 depending on condition

Meat and Potatoes - Ring and Pinion
Give Spence a call at weirperformance.com. Tell him that you want an 8" ring and pinion set for a mark 3 Supra and that you intend to use it in an IS300 or FR-S/BRZ carrier. Then just make up your mind what ratio you want. He also sells a shim kit that I highly recommend for $15. It has all the shims you could possibly need to get the install right.

Cost: $225 for ring and pinion, $15 for shims, plus shipping

Fiddly Bits - Seals and Nuts
For these I just visited a couple different Toyota parts distributor sites. Here's the breakdown of what I got for my installation:
90179-20018 Pinion nut
41231-24010 Pinion spacer
90311-43009 Side seals (x 2)
41222-22010 Ring gear lock tabs (x 5)
90311-38070 Pinion seal
Note: My carrier was basically brand new so I didn't need to replace any bearings. These tend to be rather expensive (pinion bearing I think was about ~$80, not sure on side bearings). If you think you need to do the bearings, you may as well buy Spence's installation kit. It should contain all of these parts plus the bearings. Not sure on the price there though, but it's on par with acquiring the parts individually.

Other note: If you're looking to make Balls-Out Torques, you may consider getting a solid spacer in lieu of the OEM pinion spacer. Also available from Weir, also part of the installation kit.

Cost: ~$100 without bearings, plus shipping

Putting it together - Labor Cost
4 hours for the ring and pinion install, 2-3 hours for carrier install. I'd recommend a dedicated differential shop for this just because they should be guaranteed to get the setup dead on. Labor costs vary widely, and you could easily save a couple hundred bucks by installing the carrier in your car yourself. I chose to let the Professionals handle it all.

Cost: ~$700 if the shop does everything, ~$400 if you install your own carrier, $0 if you're a Professional

Price Breakdowns
Bare Minimum (just required parts, all installations done yourself): $350
Bare Minimum + ring and pinion installed by shop: ~$750
My Total (spare carrier, no bearings, all installation handled by shop): ~$1100
Theoretical Max (all parts, all services done by shop): ~$1500

So there you have it! Let me know if you think I missed anything or need further clarification on something. Big thanks to @37, @Jeff Lange and anyone else I may have forgotten for helping me figure out what ratio I wanted and what installation parts to buy.