You'll need something that will supply the normal and nominal 5.0V for USB at around 1.0A. That should be plenty to operate your 135.
Now, you wish to run the device for 7.5 hours, and your 135 is a bit older now, and the internal battery surely won't support as many operating hours as it did when new. So best bet is to get an external device that will run your 135 for the full 7.5 hours, allowing your internal battery to be used only as a backup. I'd bet it only runs an hour when not hooked to an external power source.
So find a device that at least promises a solid 8000mAh (8Ah) of capacity. Those that show 5000mAh aren't going to cut it. There are quite a few at 10000mAh (10Ah) that, if they live up to their specifications, should keep you running through your full 7.5 hour workday. It will help if you unplug your 135 (letting it time out and shut off by itself) when making stops of more than a couple of minutes. The screen backlight on any of these units eats a fair bit of power.
Just be aware that it is not atypical for a lithium ion pack to deliver only about 2/3 of its original capacity after 500 discharge/charge cycles.