hi where i can buy memory for my MacBook Pro (13-inch, Late 2011)

"Where can I get..." questions are better answers if you give us a rough idea of where you are in the world. These are international English-language forums with millions of users worldwide. It does you little good if we recommend a vendor who cannot or will not ship to your location.

HOWEVER, if your computer is slow to boot and slow in opening apps, but runs acceptably otherwise, you do not have RAM problem. Statistically, based on thousands of reports here, it is much more likely that you have a hard drive problems. 13-inch MacBook Pros from 2009 through 2012 were fitted with relatively slow 5400rpm, 3GBps hard drives. Even when healthy, they "feel" slow, making the computer a pain to use. All the RAM you can through at the problem won't fix the slow drive model Apple used.

We can verify that if you will post a system config report.  Fortunately there is a safe, secure way to do that. 

We can quickly and within the confines of these forums help you determine if software issues are the problem if you use EtreCheck, available free from the Mac App Store here:

https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12

We can also see hard data about drive performance and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find):

When its report displays, click the "Share Report" icon from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown.

⚠️⚠️ Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.

NOTE: Changes in late 2018 to the forum software require you use the “Additional Text" icon (see example below) to embed the report into a post:

Paste the report into the resulting “Additional Text” window:

EtreCheck gives us data that helps us help you with a computer we can neither see nor touch.

I need to buy memory for my mac pro, since it is very slow ...

Avoid the XY problem.

It's very easy to determine if your Mac's performance is being limited by available memory. Please read Check if your Mac needs more RAM in Activity Monitor. If the "Memory Pressure" graph is frequently "red" your Mac will benefit from additional memory. If not, it won't. It's that simple.

The breakpoint where more RAM makes a dramatic difference in Mac speed is about 6GB. less than that, and your Mac spends far too much time simulating needed RAM on the Boot drive, and is not appropriately-responsive. More than that helps, but is a diminishing return for "ordinary" uses. (If editing, "never enough".)

Beyond that, the change that will make a dramatic difference is replacing your original rotating magnetic drive with a ≥500 GB SSD drive. Today, drives like this are available for Under US$100. Users who install one say, "It's like getting a whole new computer!" -- Allan Jones

However, you should plan on installation using an external enclosure, or replacing the internal drive cable (a known weak spot) during the upgrade to avoid hard-to-debug issues.

OWC.net (Illinois) is the most recommended site for buyers in the US. They can provide everything mentioned on this thread to date at pretty good prices. The main reason to shop there is that they pride themselves on their support, and because they support it, they will only carry items that are trouble-free. So their products are pre-selected to be well-behaved.

As Bob said, OWC is a good choice.

Also check Corsair and Crucial, they both have a specific Mac Memory brand in their lineup, and I believe they're a bit less expensive than OWC and good as well.

Those three are some of the best brands in the area. You won't have problems.

But also think of installing an SSD on your Mac. It'll probably be way more effective in terms performance then just the RAM.

As Grant said, SSD will make a dramatically change in performance, way more expressive than the RAM.

Um, where in thew world are you?

Many use OWC to buy memory upgrades.