I haven't found any 64 MB or 128 MB DIMMs yet that are composed of 64 Mbit ICs with 2 internal banks. They do exist according to the webpages of the manufacturers. These will work if the 16 MBit ICs have 2 internal banks. Most shops will not know what you're talking about. But if you insist and the shop has a sense for service and client relations, you should be able to get them.
Higher capacities are extremely difficult to find.
Companies specialized in workstation memory may have them, but they can be an order of magnitude more expensive than commodity PC shops. Things will only get worse, because of the fastly evolving memory technology and computer world. You can verify whether the type of your memory at the website of the memory manufacturer. If you have no idea who's the manufacturer, just feed the markings on the chips to Alta Vista . We tried two different LongTrail hardware revisions, but the motherboard didn't matter.