Try the auction sites
Try this link, they seem to have about every caliber magazine for the 788. Copy and paste. http://www.jocoemprise.com/rifles/remington/remington788.html
788's are about as rear as hens teeth. they were made in the 60's. just keep looking good luck. try gun shows
1000$
The rate of twist for a Remington 788 in .243cal is 1 in 9 which is one complete turn of the bullet in nine inches
Just paid $350 for 788 in 30-30 caliber.
$350.00
100-400 USD
Contact Remington's customer service.
I will first say that the Remington model 788 was made in 6mm from 1969-1979,at this time they are going for anywhere from 250-450 dollars depending on condition.
stocky stocks
$500-$600