How much would it cost me to have an architectural house plan drawn?

It depends on several items such as the plot coverage, plot ratio etc
How big is the house you want drawn

Strictly speaking its 6% in an ideal world
but in the real world its negotiable depending on the size of the development, the location, the level of involvement you want from the architect during inception to completion, the car you drive and the suite the client wear( think user here) and finally the negotiating power of the two parties involved

What if i got a house plan from the internet, how would i get it approved? Can an architect provide the necessary stamps and also produce the blueprints for me and at what cost?

The full architectural service costs 6% of the total cost of construction.
This is offered by any registered architect.
There are numerous droughts men who offer the services too. The difference is in that with the droughts-men, you will not get your house personalized to your tastes and budget. It wil most likely be someone else's taste and budget. There is a perception that using a droughts man is cheaper but not the case. Drouhtsme offer you only a partial service e.g scheme design stage which is approx. 1% of the total cost of construction e.g for a 3m house, this will be 30k. Architects offer you the full service i.e scheme design plus other services e.g detailed design, supervision. Its possible to negotiate with an architect to only offer you the same level of service as the droughts man. At this level, he will be much cheaper than the droughts man but will require signing of documentation showing the level of services offered in-case issues arise later on. Droughts-men have no responsibility to the people of the Govt. of Kenya in-case issues arise as opposed to architects who will have to answer hence more careful.