IRB’s Regulations Of The Game (http://www.irb.com/lawregulations/regulations/index.html) in Chapter 12, «PROVISIONS RELATING TO PLAYER DRESS» state:
(d) Fingerless Mitts
Coverage of the fingers and thumbs be permitted to the outer joint but no
further. The mitt zone of coverage should not continue beyond the wrist.
The body of the mitt should be of a stretch type material with the grip
material being made of a soft rubber/synthetic compound not exceeding a
depth of 1mm.
No part of a mitt should contain buttons or potentially dangerous items.
There are then no other restrictions so far. So you can possibly take any type of glove that suits your needs (probably NOT rugby gloves available from stores) and just cut the fingers (not yours, gloves’ 🙂 ).