What are you putting the risers on? A set of 2pc bases or a 1pc rail? Sure you knew this, but a Farrell base is alot higher than a set of 2pc bases and may get you the height you need if now using 2pc bases. I am assuming thats not the case though.
Only ones I've seen are the ones alot of AR guys use, but I think they make them in 1/2" and/or 1" height. Never seen a 1/4" riser myself, but that dont mean they dont make them.