Mostly luck of the draw, unless.....
There is no way that I know of, except maybe from some of the more "specialized" vendors, of requesting specific chips.
My "technique", if you would call it that, for getting the chips I want is to find the ram I am looking for (actually, the chips that I am looking for) at a retail store where I can do a "visual" to verify the chips. I then buy that ram, say Kingston valueram for example, and then for the next couple of weeks, while still within the store's return period, look for sales or MIR on the kingston, either online or at another brick & mortar store. If I can find it on sale, I make the second purchase, without caring about the actual chips, I just want to buy the same product, with the same UPC label. I return, for a refund, this second purchase of the "same" product to the place that I purchased the original ram. I end up getting the chips I want, at the sales or net of MIR price. The store where I made my original ram purchase has the same retail product to sell and is out nothing.
There have been some people that have claimed that what I do is wrong, but I don't think so because the original vendor has the same unopended product to sell as he did to begin with. To them (the stores) it does not matter which chips are in the ValueRam, their only interest is having the ValueRam in a saleable condition, unopended and undamaged in any way.
I believe that you can do the same with the Crucial w/Micron chips. I have seen them for sale at Best Buy for about $150 per 512MB stick, they are $88 at ZZF for the same stick. First, verify the product code at BB is the same as what is being sold at ZZF. You then buy it at BB, set it aside and buy it again from ZZF. When your stuff comes in from ZZF, if it is Micron with the -5B C chips you keep it and return your original purchase to BB, or if it is not MT-5B C, then you keep the original Crucial purchase (with the MT -5B C) and return the stuff received from ZZF with diff chips to BB, where you made your original purchase. As long as you are returning the same product (same UPC), then no one is out anything. What is all this trouble worth? Purchased from BB: 2 x 150 = 300, plus sales tax (depending on where you live) or purchased at ZZF: of 2 x $88 = $176, savings of $124, which is a nice sum in my book.
By the way, it is best (easier) to return items if your original purchase is by credit card or cash. Purchases made by checks can cause trouble because some stores have policies which state that they will issue refund checks from Corporate (read: slow paying) offices when the refund exceeds a certain limit, usually around $200. It can take several weeks to get your money back if the original purchase was paid by check.
Daggumit, I am LONG-WINDED, aren't I?