There's nothing wrong with Walmart foam good sir. Sure it has a textured side, but the price is right and based on the way most Pep files work you'll never have any visible texture on the external facing (and who cares what it looks like when it's pressed up against your body).
Tips for clean cutting depend on your tools honestly, what I use is 10mm snap off blades mostly, a metal straight edge and a kitchen knife block sharpener so I may be more/less chintzy when it comes to equipment, I'm not sure. Sharpen your blade every 4-5 cuts and make sure you're sharpening at a consistent angle so that you're not honing the edge into wobbly forms that'll snag on the foam. If you notice that you can't get an edge to stay on your blade longer than 2-3 cuts, attempt a more severe sharpening angle or just chuck the blade and start fresh. If you're using cheap snap off blades, buy a pack of 10 replacements for like $2 and that'll last you the whole costume if you're sharpening your blades well, for my Reach Spartan I did two blade swaps.
When working with foam, it's all about the bevels and undercuts. If there's a hashed line on the pep file make sure you add the valley cutout on the reverse side of the foam to at least half the foam depth, three quarters is preferable. Use the 3D representation of the file on Pepakura Designer/Armoursmith to your advantage and figure out how pieces meet and plan your edge bevels to make the largest possible mating surface with the least amount of backfill required. If you don't have access to a computer that can handle those programs then worst case is to piece your patterns together with painters tape, measure the angle between pieces and use that divided by two to bevel the edges. It makes for so much cleaner of a build than just winging it and jamming pieces together.
Source: Just finished my Reach Spartan assembly and had noticeable improvement between first built piece and last built piece using and perfecting this method.
