A door threshold extension does not have to be difficult to find. Door threshold extensions are very strong and durable. You can find extension door thresholds, commercial door threshold extensions as well as exterior door threshold extensions among other products. A suitable threshold is definitely necessary when you're setting up a good entry, although you have to make sure that it reaches all the way, so there are no gaps.
If your threshold is a little on the short side, then you can purchase a threshold extension and fill in the gaps. These types of units are available in a range of sizes as well as materials. So no matter whether you're looking for an aluminum door threshold, a door threshold seal, an interior door threshold or a front door threshold, you're sure to find a model that works nicely with your dimensions and your style.
Chamberlain 7806CB Screw Drive Extension Kit for 8-Feet High Garage Doors
"8' Rail Screw Drive Extension Kit Is For 8' High Garage Doors & Compatible With Chamberlain Screw Drive Garage Door Openers Manufactured After May 2007, Includes An Extension Rail, Mounting Brackets & All Necessary Hardware."
M-D Building Products 49008 Prem. Adjust/Thermal Break Threshold, 36 Inches, Satin Nickel
"Premium adjustable threshold. All season protection against drafts, moisture, dust, and insects. Thermal break design prevents frost build up on interior surfaces"
These thresholds can be found at amazon.com