Don't laugh, but check out those stores and kiosks that are in every mall in the known world where they sell cheap necklaces, earrings, that sort of thing. A lot of their stock contains MOP and abolone shells in various formats which are simlpy coated with a thin clear coat that easily sands off. Voila! The exact same shells that you can get for $20/oz on ebay (or more from other online dealers) you can get for a fraction of the cost.
My girlfriend works at one of these stores (hence why I don't mention the name, but I guarantee you that there's at least one in every major town and city in ) and it's where I get a lot of inlay material. Just last week she brought me home three necklaces that each consisted of a full shell of black MOP. Not just a small fragment, but the WHOLE shell. Just those three shells alone would cost at least $20 from an "inlay material" place, or less than $10 from one of these stores (even less if you can convince a staffer to give you their discount). You do the math.
Granted, a whole shell still has it's curve and therefore will only be partially useable (the flat parts) but, how can you go wrong for the price? Plus the act of cutting the shells into useable, flat blanks is good practice for when you actually try to cut your pattern out.
Laugh if you will, but I've got pearl shell material in black, pink and red heart, gold, white and abalone from this place, not to metion every colour imaginable of coloured plastic that looks like MOP.
Rock on