Unfortunately, door handles are made of unobtainium. Depending on how quickly you need them, there is junkyard near me that has plenty of working door handles. The problem you face with the 82-83 celica/supras is the door handles are completely different from other year Celicas. I believe that those type of door handles are also different from other toyotas as well. You can try to adapt the 84 and later door handles, but I don't believe it works. There was someone on Celicasupra trying to come up with a kit do that, but I don't think he was successful.
Depending on the trim, some may still be available at the dealer. Most will have to be found at either a JY or from some one parting one out online. Check with either Celica-GTS dot com or celicasupra dot com for part outs.
Interior parts, you will most likely be able to find a good center console, they are pretty easy. The dash will be alot of work. If you are serious about making it a show car, check with Just Dashes out in Cali. They might be able to recover or recondition your existing dash.
Carpet kits should be available on Ebay or other places. The front interior carpet should be readily available. The rear cargo area maybe a harder search. I do not know of anyone making replacement carpeting for the rear cargo area. There was someone on celicasupra that made their own and it turned out pretty well. I have reconditioned alot of the interior of my 77 with spray paint. Luckily, I have a black interior in that car. Krylon Semi-gloss or satin black looks good on the hard plastic parts. I have yet to try and see how it looks on some of the softer plastic in our interiors, such as the radio surround and such.
If you plan on showing the car, clean, clean, clean. See if you can tighten up the engine bay wiring. Look for some wire wrap. You can then bundle the wire together and tuck it up out of the way. Make sure that the wires are in good condition, no fraying or cracking. If you have questionable wiring, look at redoing the bad stuff.
Shop the local dollar store for things like toothbrushes and other small brushes. They will help get to the knooks and crannies in the interior. Pull the door panels and such and shampoo them. In my 87 supra, the fabric sections of the interior panels come out. I was able to use some resolve carpet clean to bring them back to like new condition. Just becareful when removing them as some of the connectors holding them on can be brittle.
As far a suspension adjustments go, I believe that you can get coil-overs that will work with your car. This would allow you to get it a bit lower. The only way to get really low and still retain stock geometry and such is baggin it. Again, check celicasupra dot com about suspension tricks. I believe that either 240SX coil overs or AE86 stuff may work.
Plenty of stuff to start you out there.
1976 RA29 project car, now a driver!!!!!
1985 MX72 soon to be...