Depending on the frequency, PVC's loss tangent will ruin your loop Q, so try to use HDPE or LDPE instead.
Possible source of litz wire can be transformers used in high efficiency SMPS's, but the ideal litz wire (wire thickness and qty of wires) changes, even across the MW band.
If you want to build a kick-arse receive loop for MW, I highly recommend the late Graham Maynard's "Sprial loop with Q multiplier" as found in Practical Wireless - the best loop I've ever used (feeding into a Racal RA17L).
