There are two popular kinds of inexpensive solar filters which you can buy in unmounted sheets, cut them and mount them yourself. The older kind is the black polymer type, with the most famous kind (especially in the US) being the one that Thousand Oaks makes (their glass filters are also popular), while the currently more popular (at least in Europe) type is Baader’s Astrosolar safety film. I have used the Baader film for a few years, but thought I’d give the Thousand Oaks a try in case it gives me something the Baader does not.
You can get the Thousand Oaks filter directly from the US manufacturer, or you can find a bit cheaper sheets on amazon.co.uk, amazon.com or ebay (starting at around $15/£14 for a 6″x6″ sheet). The Baader film can also be found on amazon.co.uk, amazon.com, FLO and ebay with prices starting at around £20/$37 for an A4 sheet. Be careful, we are talking here about the “visual” (ND=5.0) Baader film, as there is also the photo version (ND=3.8) which is only for high-power photo use (or visual with narrowband filters) – for a test with that filter check here.
Both materials are relatively easy to cut with scissors. Don’t worry if the baader film looks crumbled when you mount it, you are not supposed to tighten it, that’s how it should be. The black polymer seems like a tougher material, but as I’ve never had the Baader tear on me with some reasonable handling, I wouldn’t say it is too sensitive. It is always recommended to check your filters for holes before your session in any case – this particular Baader hasn’t developed any in the 3-4 years I’ve been using it.
The biggest difference when you see through these two filters is the color of the Sun: