I am told that 901 carts (which are flat/whistle tip) work fine with the 510. The only complaint I have seen is about them being a little tight, but no biggie.
Also, cignot does sell them, but it's 5 bucks for 3 (with pyramid tea bag filler) or 5 (prefilled). I am sure more sites sell them, but they seem to be few and far between.
Going the 901-cart route just seemed more convenient, as more sellers have them, and you get more for the money (I think). I expect to get em in a couple of days and I can let you know how it works.