If I need a disc right away, I get it from Express Karaoke because we are on the same coast and it will usually get here within the week. They will seem to have cheaper disc prices but it will end up around the same per disc as Ace once shipping is added. Their shipping is alot so sometimes I pay for poor planning but they also have some good sales which can make up for it. Ace is just a bit slower but I use them when looking for Latin songs as they have a large selection. Oh--Express once emailed me asking if I really wanted two copies of the same disc. I didn't. Had made a mistake. I really appreciated that bit of customer service.
Chartbuster is really fast on the shipping, even on Customs. I have been getting about a Custom a month from them to fill country requests. They have been coming in 4 to 5 days. I use Selectatrack to fill rock/pop/obscure/new song requests. When Sound Choice gets back their Customs I will use them as I have been ending up with 2 to 3 songs per Custom that aren't going to cut it--duets not different colors, etc. --it is really frustrating me.
I have also got discs from Dollar Store Junkie on line, including an SC Headbangers. But have gotten what I want from them and there seems to be few new additions.
Karaoke Warehouse I have used several times. They have had really good sale prices. If you keep an eye out on the left, a code will come up that you can enter for free shipping if you buy a certain amount. They also send out coupons for a percent off each month. But beware and make sure you don't just type in the coupon code and go on with your checkout. You have to push "submit" first or it won't take and you can't ask for your discount retro-actively. Their shipping has improved to the point that I have been getting my order within a week. I had one return of a PHM that was jarbled right out of the box and they replaced it promptly plus sent a prepaid envelope for the return. (It came with the replacment disc. I didn't have to make the return first.) I'm a bit wary of their downloads/rental options. Not sure what to make of what they are doing and if I like it.
Have also bought from LoudKaraoke, Hip2 on EBay, etc. and had good experiences but there are just so many companies I feel like dealing with so those have sort of gone by the wayside.