If you are referring to the SATA data cable (I assume you mean the flat cable), that can be purchased at any local computer shop. The SATA power plug should already be in the system as there is usually a spare. If not you can purchase adapter cables that are Molex to SATA and SATA splitter from a single male to 2 female plugs.

The following link should help.

https://www.amazon.ca/Inateck-Connection-Cables-Splitter-ST1003/dp/B01FD7R0ZM/ref=sr_1_3?ie=UTF8&qid=1545877066&sr=8-3&keywords=sata+power+connector

For main hd i see a right angled sata power plug going from the harddrive into my motherboard in a port which says "SATA PWR" and that is a black 6 pin plug which that is plugged into. Would I also need the same thing for my ssd, if so where would i be able to find a sata power to 6 pin adapter cable that fits into the "SATA PWR" slot?

http://topics-cdn.dell.com/pdf/inspiron-3668-desktop_Service-Manual_en-us.pdf

KC81G  Laptop Optical /3.5 inch HDD power cable on left by 5 and 6

https://www.amazon.com/Eathtek-Dell-3653-3650-3655/dp/B01N224R5I/

X9FV3 is the Blue 2.5 CADDY you will also need

https://www.amazon.com/OptiPlex-Inspiron-Vostro-Bracket-X9FV3/dp/B01DRV10L6/

949TY ASSY CBL SATA 2.5

GP2JM DUAL 2.5 HDD SATA Power Cable on right by 7

https://www.amazon.com/Eathtek-Dell-Inspiron-3653-3650/dp/B06WP4KMQB/

I'm trying to do the same thing.  I got an SSD for Christmas and I want to add it to my Inspiron 3668.  Upon opening it up, I was dismayed to see there's no free SATA or power cables.  After doing some research, it appears I need both + a mounting clip to go in the open bay that holds up to two 2.5" drives.

I think it's these 3 items that show up under "Frequently Bought Together"...

https://www.amazon.com/gp/product/B01LI8MZ46/ref=ox_sc_act_title_2?smid=A202B43RMN2RRD&psc=1

https://www.amazon.com/Eathtek-Dell-Inspiron-3653-3650/dp/B06WP4KMQB/ref=pd_bxgy_147_img_3?_encoding=UTF8&pd_rd_i=B06WP4KMQB&pd_rd_r=9e4d0ce1-0aec-11e9-a31e-7f34122bc55b&pd_rd_w=JuKXY&pd_rd_wg=mszwv&pf_rd_p=6725dbd6-9917-451d-beba-16af7874e407&pf_rd_r=W0593MDPTA9G7Z7SF2NF&psc=1&refRID=W0593MDPTA9G7Z7SF2NF

https://www.amazon.com/Monoprice-18-Inch-Cable-Locking-90-Degree/dp/B009GUXU52/ref=pd_bxgy_147_img_2?_encoding=UTF8&pd_rd_i=B009GUXU52&pd_rd_r=9e4d0ce1-0aec-11e9-a31e-7f34122bc55b&pd_rd_w=JuKXY&pd_rd_wg=mszwv&pf_rd_p=6725dbd6-9917-451d-beba-16af7874e407&pf_rd_r=W0593MDPTA9G7Z7SF2NF&psc=1&refRID=W0593MDPTA9G7Z7SF2NF

Hopefully someone can correct me if I'm wrong, but that power cable can be plugged into the open connector on the motherboard labeled SATA_PWR2, correct?  And that will supply the power without needing to split from another cable?

And the data cable plugs into SATA2 or SATA3?  (Does it matter which?)

 I got this with my dell Vostro 3668 as a power connector for my hard disk. It's a six pin connector to sata power connector to xxx connector. 

 I don't know what is this xxx connector dose. Seems like it use for adding additional sata power connector with it. Because my motherboard has only one sata_power port. But it has 4 sata hdd port for hard disk connect. I brought a ssd for me and new I can't able to install it. Because of power connector. In this case I like to ask few questions here. 

What is in 2nd pic? Why this is not connected anywhere on my dell pc? Can I use it as a separator jack for another sata power connector? If yes which cable I need, if no what should I do now?

The 6 pin connector is slim connector for laptop optical drive.

You would need optical bay to ssd caddy to use that.

https://www.amazon.com/Caddy-9-5mm-Genuine-Newmodeus-product/dp/B00V96BYPO

I found all the cables your going to need

so for the power supply you need a 6 pin to 15 pin sata cable

I found one on walmart

https://www.walmart.com/ip/HDD-SSD-power-cable-6-Pin-to-SATA-15Pin-converter-cable-for-dell-3668-3667-3650/237443408

Also the Sata Data Cable Hard Drive Cable

https://www.bestbuy.com/site/insignia-2-sata-iii-hard-drive-cable-black/6406731.p?skuId=6406731&ref=212&loc=1&extStoreId=483&ref=212&loc=1&gclid=CjwKCAiA-9uNBhBTEiwAN3IlNJSV_6Jra53enjdolr12aRuC_MlgxXRgrQu4qVbXl_DkzDHWSC1cehoCmIwQAvD_BwE&gclsrc=aw.ds