Hi Van, and welcome to Vogons.
The SCC-1 is a possible option, but it'll be expensive just to use it as a MPU-401 card. Furthermore, the CM-64 already has the CM-32P counterpart which has good PCM samples. But then, the SCC-1 is a Sound Canvas on a card with built-in MPU-401 by none other than Roland itself. (Hence the asking price.)
From time to time, you may come across the "PC MIDI Card" by MusicQuest. It's a 8-bit ISA card, which is MPU-401 compatible including the intelligent mode. They come with the models MQX-32 or MQX-16 series. Usually these are sold cheaper since most are not aware that it's a MPU-401 clone.
From QuestStudios, Cloudschatze has written that the following cards are "either directly contain, or are 100% compatible with, Roland's MPU-401 chipset":
CMS CMS-401
CMS CMS-444
Midiman MM-401
Music Quest MQX-16(s)
Music Quest MQX-32(m)
Voyetra V-4000
Voyetra V-4001
Voyetra OP-4001
Voyetra V-22/24
All of the above are quite rare to get nowadays.
For further information, you can visit www.queststudios.com and Vintage Computer Forum Marketplace.