Any department store carries only the best-selling titles. Cast albums do well in major cities where shows play but less well in smaller towns which have maybe one or two theatre fanatics who buy everything.
Best wasy to encourage your local Walmart or Best Buy to carry cast albums is to ask them why certain titles are not in stock, and when they are in stock, buy them there. Sales will translate into more new releases of that genre being brought in.
Best Buy does carry cast albums in some stores (some Best Buy stores do not carry CD's at all.) Media Play used to be a good chain for getting cast albums, new and old but they have really cut back on the floor space given over to show & movie music the past few years!
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com