Pierce makes microdialysis cassettes.
As for your actual problem, samples with high salt tend to be more conductive. Thus, when sample salt concentrations are way higher than the gel salt concentration, current selectively flows in the region near the sample, rather than uniformly throughout the get. Since migration depends on the current flow, samples will run faster in regions of high current flow, in this case in the region containing high salt samples.
This can be simply solved by dialyzing your sample prior to loading OR by making a gel with a higher salt concentration. High-salt gels run warmer, and may need to be run at a lower voltage than a normal gel (active cold circulation, or running it in a cold room also helps).