Sorry for the delay. We have several types of filters, including Spectrapure membranes.
Each of the pre-filter membranes (carbon blocks, diatom filters, sediment filters) are made by various companies to spec for standard units, among them SpectraPure. We've found no difference among the lab-grade cartridges, so we use many suppliers. The same is true for the DI resin. The bulk grade hasn't shown significant difference to the branded, except for specialty media like phos-buster or others.
So long as the cartridges meet their nominal guarantees for pass-through and micron size, you should be fine using any of the available manufacturers. By using in-line TDS meters, or by adding probe-ports to your unit so that you can apply a TDS meter after each stage, you can pinpoint when a cartridge is used up and replace that one specifically instead of replacing them all on an unnecessary schedule.