OEM chainrings tend to target specific standard usages (i.e. road or MTB), so the chainrings tend to be either large (road) or small (MTB), whereas a sub-compact tends to be somewhere in the middle, so OEMs may not always have the chainring sizes best adapted to a sub-compact. Aftermarket chainring suppliers tend to offer a wider range of sizes than OEM, so they may provide a better alternative. In particular,
TA Specialites and
Stronglight offer a good range of sizes, and offer matched pairs. The TA Specialites US distributor is
Peter White Cycles, who carries a wide range of sizes. Stronglight has less availability in the US. Peter White Cycles also carries some Stronglight, but European on-line retailers often have a wider range of both TA Specialites and Stronglight chainrings than US retailers (and usually at better prices, too). Two European retailers with a good variety of Stronglight and TA Specialites chainrings are
Starbike in Germany and XXCycle in France. A number of UK retailers also carry these chainrings.
As far as the larger chainrings from FSA, they have two varieties: Their 104 BCD chainrings are much like other brands 104 chainrings, where the larger chainrings (42+ tooth) are intended as the outer chainring for triples MTB cranks. However, some of the double road cranks use 120 BCD for their outer rings, and these can be mounted to an SRAM 120/80 MTB spider (using standard two-piece chainring bolts for single chainrings). FSA makes 120 BCD chainrings in 46, 48, 50, and 52 teeth. Sadly, FSA does not make 80mm BCD inner chainrings to match the 120mm BCD outers, but instead uses their proprietary 90mm BCD (smallest chainring size 30 tooth).