There are many websites around, however very few offer mixed sizes. I actually haven't seen mixed sizes except for starting a colony.
I can't remember the site, but there was one that offered 4000 crickets of various sizes... it may have been premiumcrickets.com
I usually order any feeders I need from http://www.mulberryfarms.com/cricket.htm when you add the items to your shopping cart at the bottom of the page there's a place for comments you can ask there for mixed sizes. Or you could email them admin@mulberryfarms.com and ask about mixing sizes. I know they mix sizes of hornworms for me I've never asked for mixed size crickets but I'd think they would do it by request. :wink: