I used to do this all the time but it's more difficult now. Just a general warning that this probably isn't a good idea unless you're very responsible with your money because it's easy to get yourself in a bad position if you're not careful.
You can get a new credit card that does balance transfers and request balance transfer checks from them. Then just use one of those balance transfer checks to mail a payment to the loan you want to transfer. Make sure your don't use the entire credit line as the credit card will have the balance transfer fee put on it as well. You used to be able to find credit cards with 0% balance transfer fee but I haven't seen one of those in ages. Chase Slate is the lowest I've seen recently at 2%.
Alternately, if you have a lot of expenses every month then it's easy to find a credit card where all purchases are 0% interest for a year or more and use that to pay every possible expense for a few months and use the money you'd normally use to pay for those expenses to pay off the original loan. If you're regular monthly expenses are high enough you can pay off the original loan quickly and then pay on the credit card with no interest as normal.
The banks are looking to hook you so make sure you pay them off before the zero percent runs out or make sure you know what happens after it does. Normally the rate sky rockets. Also, don't use that card for anything else. Credit card companies always put payments towards the lowest interest rate first so if you charge something that doesn't qualify for 0% then it will collect interest until you've paid off the entire 0% balance which will likely take a while and cost you a lot of money.
If you have to pay a balance transfer fee then figure out if it's less then you would have paid if you continued paying interest on the original loan.
Good luck. I hope it works out for you.