Custom Power Supply for 6v DC Motor

all you may need is a reasonably well regulated 6VDC power supply; perhaps even a variable output bench supply would work (set for 6 volts of course). You need to know how much current the wallwart is rated for; then the power supply should be capable of 25% to 30% more just to be conservative & reliable. Many electronics outlets such as Mouser, Allied, etc sell them. You can probably even find something on fleabay or Craigslist at a good discount.

if you can solder; look for a kit and build it yourself

If it were me, I would make one. If you want to buy one, check out a company named "Calex". They sell really high quality, low ripple, low voltage power supplies that come in very nice small packages. Don't know if they make one exactly suited to your needs in terms of current, but for sure they make 6V supplies. Cost around $100-$200. You might also check eBay to see whether anyone sells 6VDC regulated power supplies for tube filaments. Possibly there is a Chinese source for a cheap but decent supply. I'd go Calex.

Just checked the Calex website. They don't make a 6V DC supply per se, but they do make 5, 10, and 12V ones. You don't say how much current you need for your motor, and that may be a shortcoming with respect to Calex. They make a 1A, 5V supply and a 400mA, 12V supply. You could make a voltage divider network to change 12V to 6V, but the current may be a deal-killer (if you need more than 400mA.). Instead, check out regulated 6V supplies meant for vacuum tube filaments. Several companies sell them in ready to use form. IMO, it would be silly to pay $1000 for the supply you mentioned. Also, what's wrong with a 6V car battery? Old cars used 6V instead of 12V electrical systems. Many companies make very high quality 6V car batteries for auto and marine use (check out "Optima"; it's the best). That's your cheapest and best solution. If you bypass the battery output with a lot of capacitance, it makes a nice difference. I run my cdp with a 12V car batt; it's attached to a trickle charger which is charging when the batt is not in use.