Do NOT buy those sardine clamps.
I have replaced all of my clamps with the squeeze type Toyota clamps. However, you have to get the CORRECT one to fit over the 5/8" Green Stripe hose. It's different than most.
I bought a 100 ft roll of 5/8" green stripe from Amazon and have not used any of...
Well, the REAR heater is still in stock at aftermarket places. RockAuto, Napa, O'Reilly's, etc. And possibly Toyota.
It's the front one that isn't available and is a huge pain to get to, but fails more often than the rear one.
There is a "rear heater bypass kit" available from Wits' End. It installs on the firewall under the heater valve so you bypass those rusty hard lines as well.
@NLXTACY
Yes, there are plastic covers over the rear bolts on the front seat. You can pry them of (may or may not break them) then easily remove the seat with 4 bolts and a harness connection.
The hoses for the rear heater are fully under the truck. The rear heater is the lowest point in the cab so...
Replacing the front heater core requires removal of most of the dash.
Make sure you get the correct one for your year. There are at least (3) versions made and only one is even remotely available in aftermarket.
You may have to remove yours, have it rebuilt, then reinstall. Having it rebuilt...