Fuses are not the precise thing a lot of people seem to think they are. It is not there to go open-circuit when the 'scope consumes 1001mA from the mains - it is there in case of a major failure like a shorted turn in the transformer.
With that in mind, there is no point getting worried about this - any 1A time-delay fuse that is the same physical size will do the job.
I've just taken a look at several of mine, and they all have 1A fuses from random manufacturers. No idea if any of them are original or not.
Fuses can fail for no apparent reason when they get old. I've not yet seen it with a Telequipment 'scope, but have seen it plenty of other times.
So, if a replacement "1AT" fuse fails, you've got a problem with your 'scope. If not, don't worry about it.
I don't know which suppliers you favour, but this link should show you how many choices you have from RS:
https://uk.rs-online.com/web/c/fuses...419,4294584567
I'd probably not risk eBay and Amazon Marketplace for something like this.
If you're in any doubt about the power supply, then it would be worth using a lamp limiter while you investigate.