Have you tried a normal seamstress/tailor?
Have you considered doing it yourself? I do most of my own leather repairs and have done a couple of textile mods, too. Not always amazingly pretty, but all of them have held for years. Give it a go...
For non-stress things like clips and straps and pocket flaps, I just get the strongest button thread in the shop. We have haberdasheries, John Lewises and all kinds of places that sell this stuff.what kind of thread do you use?