Where can I buy a fan for a Nvidia Geforce 7300 GS?

If you are talking about buying an external fan to go along with the card, anything will work (depending on the type of tower/case you have.) If you are talking about replacing the fan that is built into the card, you can attach the Zalman vf1000 fan to the card & it will work just the same. Honestly, the best thing to do, because the GeForce 7300 is a relatively cheap card, you would be better off replacing the entire card with the same or newer model.

Frankly I’d rather see you toss out the 7300GS and buy a new card instead. You can get an inexpensive contemporary card for not much more than what a fan would cost and it will have about four times the performance of this old card. For example, the Sapphire Radeon HD4350, which will cost you just $24.99 after rebate and doesn’t even need a fan.
http://www.newegg.com/Store/Category.aspx?Category=38&name=Video-Cards-Video-Devices
benchmarks:
http://www.videocardbenchmark.net/mid_range_gpus.html
But if you must go the fan route, the simplest to add is a slot fan which you can install next to the video card:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835106083&cm_re=slot_fan-35–106-083-Product

I agree with @marymaryquitecontrary . Your card is really a piece of trash and it would be best to go buy a better model. The fan may actually end up being more expensive than the card.

Is there any particular reason why you can’t upgrade?

Yes I don’t have much money.

This card is only $9.99 after rebate, definitely cheaper than most fans, and twice as fast as your 7300gs

http://www.newegg.com/Product/Product.aspx?Item=N82E16814500161&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-nana_-na&AID=10446076&PID=3928400&SID=skim2157X567668

what is rebate?

You pay $34.99, and after you buy it, you fill out a form and send it with proof of purchase and you get $25 back, so you end up paying only $9.99