I use the Pferds from Bailey's. I've heard Vallorbe are OK too, but I've never seen them available. I thought I was wearing my square files out quick too, but got thinking about a few things then I quit worrying about it:
1. I use Stihl RSLK. Pretty hard tooth, hard on files. Judging by your sig, that's what you use.
2. When square filing, you use the same part of the file every time, all the time. A round file can be rotated, so you have more filing surface available to wear down.
3. Since I am not proficient yet, I find myself filing more because I'm chasing a high or low corner, or bad angles. I'm getting better, though.....
4. I think I get as many or more chains out of a square file than I do a round file. I usually get a 3-4 chains out of a round file. The last square file got 4 or more done, and one was converting round filed to square filed (that's a lot of filing). Another one of those was a 32" full comp loop. There's an hour and a half of my life I'll never get back!
5. You can buy a lot of files for the price of a Silvey Pro Sharp. So you burn a few files. Square Rocks!
Hope that helps