For Pad Thai, tamarind paste is what you need to try to find. It's a thick, tangy dark paste made from...tamarind. There are several places you can look to find it. Your best bet is to go to an ethnic grocery store (Asian, Middle Eastern, African, Mexican) if you live near one. You may also be able to find it in one of the various ethnic sections of your local grocery store (ours used to carry it but now they don't). If all else fails, and you really love Thai food, you can order it online. It's not expensive.
Note: Sometimes tamarind comes in a brick form and if that is all you can find it's fine to use that, but make the sauce in a blender instead if you have a brick of tamarind (or in a sauce pan and melt it over low heat). Also note: you can make the sauce saltier with more shoyu or tamari, tangier with more vinegar or tamarind, sweeter with more sugar, or richer with more peanut butter, but you really should add the lime and Asian hot sauce at the end.
I use peanut butter in my sauce even though Pad Thai normally has crushed peanuts in it. I don't like to use crushed peanuts because they get soggy in the left overs. If you'd rather use peanuts, go right ahead.
For the Sauce
3 Tbsp. shoyu, tamari or light soy sauce
1 1/2 Tbsp. sugar (I don't usually like to use sugar, but you need it for this sauce, this amount is greatly reduced from most Pad Thai recipes)
2 Tbsp. distilled white or rice vinegar
1 Tbsp. fresh lime juice
3 Tbsp. natural peanut butter
2-3 Tbsp. tamarind paste (there is no substitute for this)
1 Tbsp. paprika (not smoked)
For the Stirfry
2-3 Tbsp. canola or peanut oil
1 lb. firm or extra firm tofu, cubed into 1/2" (1 cm.) squares
1 large white onion, halved lengthwise and thinly sliced
5 cloves of garlic, minced (yes, 5)
8 oz. mushrooms (crimini/baby bella or button), sliced
1 red bell pepper, sliced into thin strips
1/2 c. cilantro leaves
10-14 oz. rice stick noodles, the thickest ones you can find
Lime wedges and Sriracha hot sauce to garnish
1) Bring a large pot of water to a boil. Once the water is boiling, turn the heat off and place the rice noodles into the water. Give the noodles a stir and then let them soak according to the package directions. This is usually somewhere between 7-10 minutes depending on the thickness of the noodles you are using. Err on the undercooked side because you're also going to stirfry them. It's an art, really, if you over soak them they will fall apart but if you under soak them they will be crunchy in places...do it for just the right amount of time.
3) As the water comes to a boil and the noodles soak, prepare all of your stirfry ingredients and have them ready to go. Heat the oil in a large wok or your largest pot over high heat and then add the tofu (if the heat isn't high enough the tofu will stick to the pan). Let the tofu sit and fry while you finish chopping the vegetables.
4) After about 5 minutes the tofu should be browning. Give it a toss and then add the onions and toss to combine. Let the onions cook with the tofu for another 3 minutes until the onions begin to soften.
6) Once the noodles have been drained and the mushrooms are softened a bit, add the noodles to the wok. Pour the sauce over the noodles and add the peppers. Toss to coat all of the noodles with the sauce and continue to stirfry for about 5 more minutes.
This makes about 4 servings (or more if you have a light appetite and are serving other items) and should be served with fresh lime wedges and Sriracha hot chili sauce.