You can do a WHOIS here: whois.twnic.net.tw/
A list of registrars for locals (or local businesses) is here: twnic.net.tw/english/dn/dn_01.htm
A list for the rest of us is here: neulevel.com.tw/content/tw_how.html
I went with enom: enom.com
It really doesn’t make any difference who you use. I think I remember it being cheaper on Chinese sites for .TW domain (700+NT instead of 49.95US.) Maybe the lower price is for locals, though I’m not sure.
Just sharing the info I had to dig up.