To the first part of your question, there are amp makers and makers of other equipment that use aftermarket fuses. I say this as I've read lots of reviews and came across some makes that use them. They are a small subset of this industry, and no, I can't locate the reviews but I wouldn't mislead you on this.
To the second part of your question, I'd start with something like PADIS fuses. They are a German brand who's fuses sell for about $25 a piece. I've found them to be a big step up from the HiFi Tuning Silver Star fuses that I've tried that retailed for about double the price of the PADIS.
PADIS also makes Furutech fuses which sell for more than their own. I can't say how they compare to other makes like SR Reds and Blacks, AMR fuses, or Beewax fuses but they have that low price of entry point which should make it rather painless if you want to experiment with them.
The improvements they made to my system weren't subtle and are well worth it, for me. You can get them by googling PADIS fuses and it will lead you to a Canadian site or the German site. Either way, they are sourced from Germany. If you use the direct German site, there is a drop down box for other languages to make it easy. One is an eBay site and the other is an amazon site. The vendor I used is 'diy-acoustics-24.' Shipping should take anywhere from 4 days to 8, depending on when you put your order in.
This is controversial, but there is a directionality to them, so try them both ways. One way will definitely sound better than the other. And as for a dealer decrying the efficacy of fuses, what else would you expect?
He would have to admit that what you bought isn't built to a price point or that his work can't be improved upon. Let you ears decide and if you have thin skin, don't tell anyone if it improves your system. 😀
All the best,
Nonoise
