I think there is some confusion here.
U40 and U100 are strengths or concentrations of insulin -- 40 units per milliliter for PZI (and other pet insulins) and 100 units per milliliter for human insulins.
The 30u, 50u and 100u that you mention are VOLUMES (or sizes, really) for maximum that the syringe can hold.
Most of us use 30u syringes for our cats -- also labeled 3/10cc which means it holds 3/10ml or 0.3 milliliters because our doses are so small.
For U100 human insulins - 0.3 milliliters = 30 units of insulin.
If you examine the syringes from Walmart, they are probably all designed for U100 human insulin.
The pet syringes at U40 strength are usually only available from your vet.
To use human U100 syringes with U40 insulin, you need to use a conversion chart - because the U40 pet insulin is dilute compared to human insulin.