C64 question

Okay, I picked up quite a haul for $20: * Commodore 64 in almost-new looking condition, original box * 1541 Disk drive, in original box (but no disk drive manual :( * 1525 Printer, in box * Programmer's Reference Guide * User's Guide * Assorted software and disks * VicModem * Two joysticks * All cables Anyways, hooked it up, wrote a few BASIC programs, ran a couple of the programs supplied, cool. Now the question is, where on earth will I find some key programs -- mostly games (after all, games were its specialty!) but I REALLY want a 6502 assembler like Merlin, and to find connectors to fit the cartridge slot to hang some custom hardware off of and develop driver code for. I'll be careful. :) And if I find it somewhere on the 'net, how to get it into the C64? Has anyone solved that one? I know it's got an RS232 port, so perhaps I could write a small BASIC program to bootstrap in the program at low speed and develop something like an Xmodem download with the downloaded assembler. Just wondering if anyone has a better way to get software from 'net to C64, and knows a good source for same. Maybe a cave full of cartridges like the Atari has exists out there... :) -- Ross