As already indicated, you have several options. There are pros and cons to each.
If you are only short a few points, a good option is to purchase One Time Use Vacation Points from Member Services. These are $15 ($13.33 plus $1.67 tax) per point. These points can be mixed & matched with your points, so they're very flexible. If you're only 2 or 3 points shy of booking a night, they're a great solution. At $15, though, they're expensive. If you need a lot of points, another option may be more cost adventageous to you. You are limited to 24 One Time Use Vacation Points during any given use year.
Another option, if you want to deal directly with Disney, is to purchase cash nights. If you book your cash nights through DVC Member Services, they can be linked into a points reservation all under the same reservation number. (The cash and point nights will be seemless, and you won't have to check out/check in. There also won't be an issue if you're using a dining plan.) If you purchase cash nights through DVC Member Services, you'll receive the same maid service schedule as your points reservation. The DVC Member Services price is based on the full rack rate for the room. (The rate you see if you search for the room on the disneyworld.com web site for a quote.) A 25% DVC Member Discount may be available. There are a limited number of discounted rooms offered at any given resort on any given night. The only way to know if the discount is available is to call Member Services.
Another option is to purchase your cash nights through the Central Reservations Office (in other words, not through DVC). In this case, you'll open yourself to the possibility of better discounts and will receive daily maid service. However, cash reservations made through CRO cannot be linked to a DVC reservation. You can explain your situation to the front desk when you check in, and they may work with you to hopefully keep you from having to physically change rooms between the two reservations. However, that won't be guaranteed. In any case, you will have to check out and check back in. If you're using a dining plan, they will not be connected. Your DVC dining plan will only be valid on your DVC dates, and your CRO dining plan will only be valid on your CRO dates.
Your next options involve renting points from other DVC members, rather than purchasing your stay directly from Disney. These are private transactions between two individuals, and Disney will not intervene if you have a problem.
In the case where you only need a few points, the most adventageous solution would be to find a member willing to transfer you the number of points you need. This allows you to use their points in combination with your points to make the booking. You have a single booking under your membership number. The limiting factor with this option, though, is that DVC only allows members to have one transfer transaction per contract per use year. If you only need a small number points, you're going to be hard pressed to find someone who is willing to use up their one annual transfer to send it to you.
The other option is to rent a reservation from another member. You make a reservation in your name using your points for the nights you have enough points for, and they make a reservation in your name with their points for the other nights. These two reservations can then be linked so that you don't have to do a check-out/check-in between them. The down side here is that you don't have 100% control over the 2nd resevation, and need to work through the other member to manage it. Also, each night of the reservation must be either 100% your points or 100% the other member's points.
For either of these rental options, you can either look for a suitable renter here on the DISboards, or you can use a service like David's (one of the DISboards sponsors... look for his "wizzard hat" advertisements here on the boards) to broker the deal.
