The Free Record Shop where I bought my card really sold it ;) for other shops - they should sell as well if they're mentioned on the *bliep website.
If you want to be absolutely sure you can always just make a phone call and ask.

With kuba's help I got one sim card. Now I am using the 50 cents option, the net is quite ok, speed is decent, overall I am satisfied!

*bliep now roams in most European countries
 

Also data roaming:
- 10 MB - 24 hours valid € 2
- 200 MB - 7 days valid € 15
- without bundle € 0,54 per Mb. (billing per Kb.)
So it is easier to extend the validity of the SIM by just using the browser for a moment.
More info: http://www.bliep.nl/static/pdf/tariff.pdf (in Dutch, sorry!)

I have just returned from a week in the Netherlands and found the Bliep card very good for data. My notes are as follows

- I tried to purchase on-line. The help section suggests you can pay by Paypal - but the only options offered are Dutch banks, so I didn't succeed in purchase.
- On arriving in Delft, Free Record store had gone out of business and Mobile Phone House was out of stock. I got the SIM card in GSM Shop, Delft
- The price was €15 for the starter pack - I only needed around €7 of credit for 7 days, but there seemed to be a minimum of €15.
- The purchase was very easy - no passport/ID needed. I walked out of the shop with SIM in less than 5 minutes.
- I had to set up the APN manually, as internet.arta, User/Password - blank.
- In order to activate the card, I had to send an initial text
- I changed tariff to the €1 per day-as this offers higher speed Internet.
- The card allowed tethering - we had 3/4 devices tethered.
- The network coverage and performance was excellent

Don't forget to switch to the €0 per day tariff when you leave the Netherlands. Otherwise each day credit (50 cent or 1 euro) will be lost, even if you don't use it. SIM card is deactivated if you don't use it for more than 6 months. Sending 1 SMS (10 cent within EU) each 6 months is sufficient.

I found locations for Phone House and Free Record Shop in Amsterdam but is there a chance there's a store at the airport (AMS) or at the train station?

No unlimited data anymore from April 2015!
 

Link: https://getblieper.bliep.nl/yellow.html

Some background:
Bliep will leave the T-Mobile network and move to the Tele2 network which is 4G (800/2600MHz) only with national roaming on T-Mobile for 2G/3G.
From the 2nd of April onwards, you won’t be able to use T-Mobile top-up vouchers on the new *bliep SIM.

Can I get this sim on Thursday in Hengelo or Enschede?

No, nowadays you can only get this sim via the Bliep website. You can order a free sim (without credit) here.
I am not sure if they will sent it to Germany, but you can give it a try :)